Mangaia (traditionally known as Auau Enua, which means terraced) is the most southerly of the Cook Islands and the second largest, after Rarotonga. Geologists estimate the island is at least 18 million years old, making it the oldest in the Pacific. It rises 4750 m (15,600 ft) above the ocean floor and has a land area of 51.8 km2. It has a central volcanic plateau and, like many of the southern Cook Islands, it is surrounded by a high ring of cliffs of fossil coral 60 m (200 ft) high, known as the makatea. The highest point is Rangi-motia, 169 m above sea level, near the centre of the island. Lake Tiriara is a body of fresh water in the south. The population of Mangaia comprises about 700 people (2006). The capital is the village of Oneroa, on the west coast, containing about half the population. There are two more villages, Tamarua in the south and Ivirua in the northeast.
By plane - Air Rarotonga  flies from Rarotonga to Mangaia 3-4 Times a week. It's a short 40 minutes flight with an Embrear Bandairante. There is a baggage limit of 16kg per passenger.
For the adventurous, the cheaper option is to go by boat. The "freighter" (let's call it a nutshell) Maungaroa II goes irregularly to Mangaia. Ask at the office of the inter-island shipping company in Avarua harbour in Rarotonga. Depending on the winds it takes anything between overnight and 2 days. Be prepared to be delayed several days, don't count on a fixed departure and arrival, because it leaves when they're finished loading, the winds are good, the captain is sober and the number of octopuses in the harbour plus the number of coconuts in muri equals the stars seen at 11pm at night, no matter what they tell you when it is supposed to leave.
There is not much to do on Mangaia, except walking around, having a coconut, talk to the friendly locals and go fishing.
Fishing inside or outside the reef is very rewarding, and there is no ciguatera here unlike in Rarotonga. Spearfishing outside the reef is possible as well. There are some very nice secluded beaches, but the reef is to shallow for swimming. With a little common sense it is, however safe to swim and snorkel outside the reef when the sea is calm.
Mangaia is also good for walking.
You can get basic products at Babe's shop in Oneroa. They also have fresh bread daily (except Sundays).
Accommodations usually provide food. Self catering is also a good option if you fish. Bring as much non-local stuff like butter, eggs, meat, spices,... as you need. These things are expensive on Mangaia.
There is no internet on Mangaia, but there is mobile phone reception.