The Akiyoshido Cave will blow your mind. It is stunning and like walking through the movie Alien. It is in fact 300 million years old.
And Akiyoshido is still under construction - it feels like an underground Grand Canyon with a raging river going by you. Akiyoshido (秋芳洞) is part of the Akiyoshidai Quasi-National Park near Mine City in Yamaguchi, and is the largest limestone cave in Japan. In fact, it's one of the biggest in East Asia that is open to the public. It stretches over 9km long (only 1km is open to explore, however). It's almost impossible to wrap your head around how acidic water created the caverns over your head through eons, at the rate of about an inch per hundred years. The stalactites from the ceiling grow about 3cm (1.2") every 200 years from limestone water seeping in from the ceiling, and the stalagmites from the floor grow about the same every 4 centuries.
Above the caves you can also walk over Akiyoshidai, which is a mammoth karst plateau - basically the remains of a huge coral reef hundreds of millions of years ago. Around half a million years ago the area used to be heavily forested, and the cave was first discovered about 650 years ago.
This is one place that should be near the top of the list of any visit to Japan. But sadly, this place too is ignored or mostly unknown to the "experts" writing books on what to see.
It is one of the Top 3 caves to visit in Japan. If you love this place, you'll also love the Ryugado Cave in Kochi, Shikoku (also one of the Top 3) - see it at:https://youtu.be/eXClyRaiuio
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