Ioki Cave

Ioki Cave is a natural sea cave located in Aki City, Kochi Prefecture, on the island of Shikoku, Japan. The cave is a 40-meter long natural tunnel that was carved out by waves millions of years ago, with walls that still display fossilized shellfish, remnants from when the entire area was submerged beneath the sea. The geological formations date back approximately three million years, making this site a window into the region's ancient marine past.

The cave's fern colony was designated as a National Natural Monument in 1915, recognized for its exceptional biodiversity. Approximately 40 species of ferns, predominantly tropical varieties, thrive on the cave walls and throughout the canyon beyond. The temperature remains around 20 degrees Celsius year-round, creating ideal conditions for this rare concentration of fern species. The dense vegetation covering the steep canyon walls creates an atmosphere that visitors often compare to scenes from animated films.

Walking through Ioki Cave takes visitors from the dimly lit tunnel entrance into a lush canyon where a small river connects three waterfalls hidden among the green fern forest. The complete walk through the cave and canyon typically takes 20 to 50 minutes depending on pace. The trail follows the stream over rustic bridges, with the largest waterfall located at the end of the route, where water collects in an emerald pool. The unpaved, natural trail can be damp and uneven, so waterproof footwear is essential.

The cave entrance is located just a 5-minute walk from Ioki Station on the Gomen Nahari Line, which is approximately a 1.5-hour train ride from Kochi City. Visitors can borrow rain boots from the local Ioki community center near the parking area, though availability varies and larger sizes may not be in stock. Optional guided tours are available for ¥1,000 per person, though these are conducted only in Japanese. The site is open year-round and accessible to visitors of all ages. Parking is available across from the cave entrance.