This article is a travel topic
Hot springs are natural features resulting when ground water is heated (sometimes far beyond the level of human endurance) by geothermal forces and brought to the surface, typically becoming diluted with cool surface water on the way. Many are in attractive locations and are scenic (e.g. the geysers of Yellowstone National Park, Kamchatka, New Zealand, etc.) or celebrated (e.g. the original town of Spa), hence are attractions or even destinations in their own right. However, for the hot-spring aficionado, the greatest pleasure comes not from just looking at the spring, but from getting into the water for its therapeutic powers, not to mention just because it feels good -- really good. This article will help travelers get the most out of their hot-spring experiences world-wide.
"Developed" versus "wild" hot springs, and what's a spa?
The term "hot spring" means different things to different people, and it's a good idea to know just what manner of hot spring you're bound for at a destination, because it may be something quite different from what you're expecting. In English the term is used more or less interchangeably for "wild" springs, where the water emerges directly from the ground into a natural catchment that can be used for bathing, and "developed" springs, which exploit the spring through construction of man-made artifacts such as pools and bath houses. "Wild" springs and "developed" springs can differ so much, and in so many ways, that the visitor expecting one and getting the other may not enjoy the experience very much. Some examples:
Note, incidentally, that a "developed" hot spring is not necessarily a commercial hot spring, i.e., one that has been developed for profit-making purposes. The distinction can be important in countries and regions where the political/economic system allows for both for-profit and public-interest/non-profit/governmental development; regulations for doing the developing will often differ between the two cases, as will the resulting amenities, access, etc. For example, as a general rule, springs in the United States that have been developed by government will have fewer amenities, but also lower admission fees, than for-profit developments. In Japan, many hot springs in rural locations are maintained by the local government and are open to the public for free, and even expensive spa resort towns usually have at least one public bath open to all for a token fee.
There is a difference between a hot spring and a spa. The latter term denotes either a pleasantly warm tub of water (not necessarily originating in a hot spring) suitable for bathing for medicinal and recreational purposes, or the -- sometimes incredibly elaborate, luxurious, and expensive -- resorts where such tubs can be found, which incorporate massage, body wraps, and so on. Not every spa is based on a hot spring (many, perhaps most, simply heat meteoric water to the desired temperature); not every developed hot spring has spa-like amenities.
Where to find them
Indonesia is a highly volcanic archipelago and consequently has hot springs (air panas) all over the place, but few are developed or on the tourist trail. Bali is the most popular hot spring destination by a mile, but many of them are considered holy and have been developed into temples, where the locals come to bathe (fully clothed) but foreigners may not be welcome. A few, however, have been developed and are open to all, such as Air Banjar near Lovina, where stone mouth carvings allow hot water to pass between pools which are set among a lush garden.
Japan is very active geothermally and onsen hot springs dot the length of the entire country. The Japanese love their baths — so much so that the Japan guide has an entire section devoted to the topic and a visit to a Japanese hot spring is a highlight of any trip. A good search site for onsen is Onsen Tipster.
The Official Top 3 most famous hot spring resorts are Atami (Kanto), Beppu (Kyushu), and Shirahama (Kansai). Dogo Onsen makes a solid claim to be the oldest and Hokkaido's Noboribetsu claims to be the largest, while secluded hot-spring hideaways can be found in places including Iya Valley (Shikoku), Yagen Valley (Tohoku) and Oku-Hida (Chubu). Japan's major cities also have hot spring areas within striking distance, with Hakone and the many hot springs of Gumma being popular getaways for Tokyoites and Kobe inhabitants nipping across the hill to Arima Onsen. Last but not least, at the northeastern most tip of Hokkaido is Shiretoko National Park and its remarkable Kamuiwakkayu-no-taki, a hot spring waterfall thought by the Ainu to inhabited by the gods themselves. A type of spring in Japan is called Jigokudani, where snow monkeys wash and bathe themselves
Sungkai, Perak; Pedas, Negeri Sembilan; Selayang, Selangor;
There isn't much of a 'hot spring' tradition in Nepal, but the residents of the aptly named Tatopani (tato = hot, pani = water, the Nepalese word for "hot spring") quickly realized that trekkers on the Jomson and Annapurna Circuit trails would pay for a hot soak. When soaking or swimming in Nepal it's best to have a swimsuit and sarong for modesty.
On the way to Rasuwa fort, a borderpoint with Tibet located at the north-west of the Langtang valley, there's is a small and beautiful hotspring. It's completely natural, without any items. Ask the locals for the exact location.
Hotsprings were recorded to have been discovered by the Spanish Catholic priest and eventual saint Pedro de Bautista in 1589 in Laguna province near the dormant volcano Mount Makiling, specifically in the Pansol area of Calamba City and nearby Los Baños City. One example is the popular private resort in Pansol called Neruda Hotspring Resort, named in honor of the poet Pablo Neruda.
Koreans also love their oncheon hot springs and Busan's Hurshimchung spa is a creditable contender in the heavily competed contest for the largest spa complex in the world.
Being located on the Pacific Ring of Fire, hot springs can be found all over Taiwan. Popular places for bathing near the capital Taipei are Beitou, Wulai and the Yangmingshan National Park. At Guanziling near Chiayi, you can even sample hot mud springs.
My An Onsen Spa on the outskirts of Hue.
The states of Lower Austria, Styria, Carinthia and Salzburg are crossed by a tectonic fault, resulting in a large number of hot springs in the area. Most springs are commercially exploited. Towns with the prefix "Bad" in the name, such as Bad Gastein normally have hot spring facilities, which are usually accompanied by pools, saunas, rehabilitation centers and hotels of high standards. Wild springs do exist, but are more difficult to find.
There are many hot springs, traditional baths, spa and wild hot spring in Greece. Among them at Thermopyles (site of a famous ancient battle). It's free (un attended, hence with some trash around), used by he locals, water 42c. Behind the gas station on the way to the thermopyles battle monument.
A day trip from Athens, you can visit Vouliagmeni Lake and Health Spa, a lightly-developed warm spring. Even in the cool of winter, the water remains warm enough to swim.
In the center of the Santorini lagoon, just off the smallish island, there is a hot spring in the shallow seabed. Local boatmen will know about it - it isn't publicized,
Iceland sits on the fault between the North American and European plates, which are slowly moving apart, and is home to the original "Geysir"; so it should come as no surprise that it has geothermally heated water aplenty. Public heated swimming pools and attendant "hot pots" are commonplace throughout the country. The Blue Lagoon near Reykjavik, with its vivid, translucent blue, 100°F/40°C waters (cooled from their original temperature after being being pumped from a mile underground and powering an electrical plant) is a popular destination.
Look for scattered hot springs in the volcanic southern part of the country. The island of Pantelleria has well-known thermal baths, and Ischia is famous for the springs there, some of them "subaqueous" (submerged in the sea). The island of Vulcano features a "mud bath" in which the springs manifest in a goopy mud reputed to have therapeutic properties, as well as more subaqueous springs along the adjacent beach where mud-bath users can wash off the mud.
Băile Herculane – Inside the town and up the Cerna river a number of open pools with hot spring water can be accessed without charge. Locals of all generations mix with Romanian and international tourists.
Oradea – The region around the city has a number of commercial spas that appear to be developed from hot springs.
In Serbia is located world hottest spring with healing teromomineral water. World hottest healing teromomineral water is hot water spring in Vranjska Banja (110°C). Special hospital for rehabilitation Vranjska Banja
You can find working Moorish Baths, usually from natural hot spring sources, throughout southwestern Spain, including Granada. Some of these include deep soaking pools or the more traditional steam room style.
There are some breathtaking uncommercialised natural hot springs near Valencia where you can swim all year round. Access is not easy because the hot springs are located in a mountainous and rural area, but you'll find tours operators in Valencia that will take you there for a reasonable price.
The famous hot springs at Bath (England) have been used by humans since Roman times, and probably long before, but until recently were in a state of disrepair and inaccessible to bathers, although still open for viewing. Recently the Thermae Springs have re-opened as a developed spring in which visitors can bathe (fee).
Pamukkale - Meaning "cotton castle", these natural travertine hot springs provide stunning natural beauty and wonderful warm water to soak yourself in. The bright white calcite deposits make it easy for visitors to walk along the cascading water (though officials are constantly moving boundaries and walking areas to preserve the area). Most of the pools are just large enough set your feet into and only a few of the larger pools go past your knees.
Hierapolis - Five minutes further up the white waterfalls of Pamukkale you'll find the Hierapolis hot springs where you can soak among sunken Roman columns (for a 20 Euro fee) submerged in an ancient pool.
Hot springs in Canada cluster in geological settings similar to those of the United States (below). Banff National Park and Jasper National Park have well-known springs that are readily accessible to the visitor, and there are a number of hot-spring sites in British Columbia including those that are managed, such at Radium Hot Springs and Fairmont, and natural, such as at Tofino and Fairmont.
This highly volcanic yet visitor-friendly country has several hot springs, with a concentration at the small town of Fortuna due to the proximity of the active Arenal volcano. Tabacon near Fortuna is a well developed spring/spa, with several options for visits. Another area where Hot Springs and Volcanic Mud Springs are usual to see in the Rincon de la Vieja National Park area. Rincon de la Vieja National Park is full of volcanic activity.
Mexico is a volcanic and seismically-active country, and there are likely to be hot springs all over the landscape despite its aridity. A few commercially-developed springs are near Puebla, but their status following the re-activation of nearby Popocatepetl volcano is unknown; if you have information on springs in this area, please update this. Most the hotsprings of Mexico are in the Central interior states of Guanajato, El Ojo Caliente near the community of Ramon Corona in the state of Durango, and Chihuahua. The most accessible for tourists are those in the Copper Canyon area. Baja California has a number of hot pools and hot springs; San Carlos, near Ensenada, Guadelupe Canyon west of Mexicali, and the seaside springs at Puertecitos are well-known.
Most hot springs in the continental United States are located west of the Rocky Mountains, but there are a few in other, sometimes unexpected places. Quite a large fraction of the springs are in wild areas and entirely undeveloped, sometimes reachable only by a testing hike. Commercial springs vary wildly in degree of development; a few have been turned into posh, expensive resorts, but more are at the "rustic" end of the scale.
Alaska is intensely volcanic, and wild springs abound; however, most are seriously inaccessible. Developed hot springs exist near Fairbanks. There are also some derelict developed hot springs near Fairbanks which can make for an interesting hike, but beware of the legality.
Commercial hot springs are found at the unincorporated "town" of Tonopah, about 50 miles west of Phoenix. There have been recent changes of ownership.
Public hot springs are located along the east side of the Eastern Sierra, many near route 395, in Bridgeport (Travertine, Buckeye), Mammoth (Hot Creek), and others. Deep Creek is located in Apple Valley, near the town of Victorville.
The town of Calistoga at the northern end of Napa Valley was founded as a geothermal hot springs destination in the early 1860's and is still recognized as Northern California's leading hot springs resort town. Today there are thirteen hot springs resorts, hotels and day spas using hot springs mineral water in their spa treatments. Calistoga Chamber of Commerce and Visitor Center, 1133 Washington St. Calistoga CA 94515 707-942-6333 / 866-306-5588 . The town's famous symbol of active hot springs is the Old Faithful Geyser of California, one of only 3 in the world which erupt on a regular basis. Old Faithful Geyser of Ca., 1299 Tubbs Lane, Calistoga CA 94515 707-942-6463 
Wilbur Hot Springs 3375 Wilbur Springs Rd Williams, CA 95987 / (530) 473-2306  in Williams, (California) is famous for it's healing waters, historic hotel, it's 100% solar powered energy, and world renowned nature preserve  A healing experience with waters unlike anywhere in the world.
Information about these can be found in the book, "Hot Springs and Hot Pools of the Southwest" .
Although not as volcanically or seismically active as its neighbors, Colorado has a number of hot springs. Major commercial springs are at Glenwood Springs (an enormous outdoor pool/spa complex) and Steamboat Springs, while smaller commercial operations are at Alamosa and in the Salida vicinity. A wild spring familiar to thousands of "peak baggers" who climb the state's highest mountains is Conundrum Hot Spring, near Aspen and not far below the summit of 14,000-foot Castle Peak. The weekend crowds at Conundrum can be substantial considering the length of the hike required to reach it; if you're bound for Conundrum, go during the week (and be vigilant for signs of altitude sickness, as the spring itself is over 11,000' in altitude).
Where to Soak in Idaho?
Idaho alone has over 130 hot springs soaking opportunities - that's a lot to choose from! To help simplify the matter, this website organizes hot springs into 2 categories: http://www.idahohotsprings.com/
Montana has deep thermal underground rivers under a large percentage of its large expanse. The largest swimming pools are at Fairmont Hot Springs outside of Butte, Montana. Quinn's Hot Springs outside of Paradise, Montana, is unique in that the water flows from tub to tub and cool, so that each pool is a different temperature. Bozeman Hot Springs is very popular. Norris Hot Springs, west of Bozeman offers organic food and live acoustic music poolside. One hot spring outside of Lolo is without any facilities and people decide their own dress code. Old fashion Saco Hot Springs is popular with hunters. Hot Springs, Montana, has a couple different pools and claims that the water has healing properties. Old fashion Chico Hot Springs is near Yellowstone Park.
Nevada has a surprising number of hot springs considering its arid climate and lack of obvious volcanic/seismic activity. In fact, the slumbering earth is an anomaly in the geological history of the state, and there's plenty of geothermal energy available to drive the springs. Commercial springs cluster mainly around the Carson City/Reno/Lake Tahoe area, some of them quite extensively (and expensively) developed. The state, much of which is federally owned, also has a number of "semi-wild" springs -- sites on federal land that are at the end of a dirt road, haven't been developed beyond maybe an impoundment and a bench or two, and are freely available, yet are not as difficult to reach as wild springs in other areas that require a significant hike to reach.
New Mexico is volcanic country, and both wild and commercial hot springs can be found in the state. The North Central region has commercial hot springs at Ojo Caliente , a small town in the Española area, and at Jemez Springs in the Jemez Mountains. The area also has several undeveloped springs north of Jemez Springs along NM Highway 4, and there are a few wild springs in the foothills of the mountains near Taos. The Southwest region of the state also includes both developed and wild springs. The town of Truth or Consequences was formerly named "Hot Springs" because of its several springs with resorts. "T or C," as it's known locally, has undergone rebirth following some years of atrophy, with about ten commercial establishments offering soaks and spa services. Undeveloped springs are found in the wild country near Silver City, and Faywood Hot Springs, between Silver City and Deming, has undergone a major transformation and is now a premier destination area. They feature private and public pools, clothing optional and clothing required, with camp sites, RV spaces and a few cabins. (Note that undeveloped springs in this region have been implicated in at least one fatal case of PAME; see "Stay healthy" below, and be careful.)
Cougar Hot Springs (undeveloped) is located an hour east of Eugene, Oregon, near Cougar Reservoir.
The Bagby Hotsprings are located in the Mt. Hood National Park. These facilities are free and natural, but you must hike about a mile to get to them.
Texas is not as active geologically as most of the American West, but Big Bend National Park includes the ruins of Hot Springs Village, with what one might describe as a "feral" spring -- one that was commercially developed at one time but has been abandoned and is returning to a natural state. Hikers can take advantage of an impoundment that survives from the time of the village; water temperature around 105 F.
The famous Homestead Crater in Midway, Utah is a geothermal spring hidden within a 55-foot tall, volcano-shaped limestone deposit. Over 10,000 years in the making, water superheated deep in the earth percolated upward and picked up minerals, which were then deposited on the surface — eventually forming The Crater. The hole at the top of the dome lets in sunlight and fresh air while the interior stays heated by the mineral water at a constant range of 90 – 96 degrees Fahrenheit. Venture inside for a one-of-a-kind experience — whether swimming, scuba diving, snorkeling, a therapeutic soak, or even take a paddleboard yoga class. The Crater attracts diving enthusiasts as the only warm water scuba diving destination in the continental United States.
The presence of hot springs in Wyoming should come as no surprise to anyone familiar with Yellowstone National Park and its amazing assortment of geothermal features, but the sites there are better suited to photography than to immersion. The springs within Yellowstone are generally off limits to bathing, and the prohibitions are vigorously enforced, at least in the main tourist areas. They are also dangerously -- lethally -- hot in many cases. People (and countless animals, including pets) have died from falling into some of the features at Upper Geyser Basin just minutes from Old Faithful and the visitor center. Yellowstone is not a place to tempt fate in a hot spring; don't enter the springs themselves, period. However, swimming is allowed (but not encouraged) at the Firehole Cascades swimming area, a section of the Firehole River that is warmed by hot springs.
Commercial springs exist at Cody, Thermopolis and a few other places. The Thermopolis spring is notable as one of the world's largest, and also because a 19th-century treaty(!) dictates that one fourth of the spring's output remain free and available for public use rather than being captured by for-profit enterprises; the park preserving it therefore offers the rare treat of a "developed" spring (multiple bathing areas, changing room, etc.) that doesn't cost anything to use it. Wild springs most suitable for bathing are reached by trails along the Rockefeller Parkway connecting Yellowstone and Grand Teton National Park, among other locations.
Innot Hot Springs on the Atherton Tablelands has hot water coming through the ground into a creek. The creek can flood in the wet season.
The Peninsula Hot Springs in Rye are an hour's drive south of Melbourne. The hot water is artificially pumped out of the ground to feed a selection of japanese-styled bathing pools.
Most of the volcanic regions of New Zealand have hot springs, some are in their natural state, and many have been developed into quite elaborate systems of pools. There are entire swimming complexes built around the spring water.
Rotorua has a large complex, and some natural swimming spots if you know where to look.
Caldas Novas has the country's largest springs (so large it's called the Hot River in Portuguese) and many hotels.
As a faily volcanic and VERY seismic country, Chile has several hot spings. They're mostly located in the Center and Nearest South regions
How to get there, get access, and get wet
Step one in locating a hot spring is knowing exactly how to say -- or read -- "hot spring" in the language of the country you're visiting. Some translations into other languages:
(Please expand this list if you're fluent in a language not yet represented here.)
Access to "commercial" hot springs is a simple matter: you do what the proprietor asks (likely involving the exchange of money, and possibly requiring reservations), then you use the spring. The situation is less clear-cut with "wild" springs. If a wild spring is located on private property, chances are good that the land owner will have the legal right to control access to the spring, including charging for entry if he/she wishes. If the land owner chooses to assert this right, honor it and do not trespass. Natural features the world over are being placed off limits as a result of trespassers abusing access that land owners had previously afforded visitors, with limitations. Please help fight this trend by respecting private property where applicable.
Springs on public land pose the most complex access issues. In the United States, a general but by no means universal rule of thumb (always inquire locally) is:
If you have experience that allows you to give comparable guidance on other countries, by all means add it here.
Etiquette, customs, and staying out of jail
What to bring with you
Clothing Optional Springs
At both commercial and wild springs, customs (and laws) vary substantially as to whether you're required to wear a bathing suit. Most commercial establishments will post their own rules, which may be "swimsuit required," "swimsuit prohibited," "clothing optional," or "required until sundown, then nudity OK" (this is commonly seen in commercial hot-spring operations in the United States). Normal practice at wild springs is much more a local, ad-hoc matter. As a general but by no means universal rule, in the United States, you should plan on wearing a swimsuit at springs in sight of a road unless the spring is specifically posted as accepting naturism; in the backcountry, you're on your own. It's wise to err on the side of conservatism, however, as some areas prohibit nude hot spring bathing and enforce the prohibition with fines. In most of Europe there are no laws regarding nude bathing, so it's up to you to decide. Local etiquette varies from spring to spring and country to country. If you know the standards and regulations in other countries, please expand this section.
Anyone offended by clothing optional use, most often including families and groups with children, should always scout locations ahead of time if the customs or practices at the spring aren't clear. Conversely, if you arrive at a spring and find a group with children, and everyone is wearing a bathing suit, you should either wait for them to leave or ask politely before disrobing.
If you do wear a swimsuit, it's a good idea to rinse it out shortly after use. The water in many hot springs is acidic to some extent and if allowed to dry in your suit it may damage the fabric.