Oboke and Koboke
The lyrical names of Oboke and Koboke have given rise to much comment over the years: respectively, they mean "big steps dangerous" and "small steps dangerous", an apt description of the perils of traversing through these steep gorges on foot.
These days the main danger in getting to Oboke and Koboke is to your wallet's health, as they're located in the middle of nowhere. National Highway 32 careens through the once-pristine valley, as does the JR Dosan Line from Kochi to Takamatsu via Ikeda. A tunnel also connects Oboke to the western Iya Valley (4 buses per day).
Both Oboke and Koboke have train stations.
See & Do
There's one main reason to come to Oboke and Koboke: 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 Yoshinogawa 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.
If you survive the experience without developing acute hydrophobia, both Oboke and Koboke have onsen hot springs offering more bubbling water to soak in.