Oboke and Koboke
Ōboke (大歩危) and Koboke (小歩危) mean, respectively, "big steps dangerous" and "small steps dangerous" — an apt description of the perils of traversing through these steep gorges on foot.
It is recommended to travel in this area by car. If you get there by train, you'll have a long walk along a noisy road without sidewalks. It is also hard to find entry points for hiking trails without a car. Unfortunately, the nearest car rentals are located quite far away: Marugame, Takamatsu, and Tokushima.
National Highway 32 winds along the Yoshino River through the pristine valley. Coming from Osaka, Kobe or Tokushima, take the expressway and exit at Ikeda. Then follow route 32 towards Kochi for about 20 kilometers to Oboke.
A tunnel also connects Oboke to the western Iya Valley.
Buses to the Iya Valley run sparsely, especially on weekdays (around 2 per day).
See & Do
There's one big reason to visit: the rapids rushing through the gorge below. There are observation platforms on the highway and near the train stations if you're content to gaze from a distance, but more daring types will venture down onto the Yoshino River for a sightseeing cruise (¥2000 for a one-hour loop), and more adventurous types yet will try their hand at white-water rafting in the class 4 rapids. The rapids are Japan's biggest and most exciting, with amazing rock formations and crystal clear waters.
If you survive the experience without developing acute hydrophobia, both Oboke and Koboke have onsen hot springs offering more bubbling water to soak in.
Eat & Drink
The local specialties are ayu and amego, delicious river fish salted and barbecued on a stick.