What an awesome photo. I actually read a small article on here about Akiyoshido Cave but I don't remember seeing this in the video. It looks like a really cool place to visit if I ever head out to Yamaguchi so I'd definitely love to check it out. I'm not sure if this is some sort of illumination event, but it looks like it has had some in the past so I'm assuming that's what this is. It's pretty crazy there's such a large limestone cave in Japan, but then again Japan does have a lot of mountains and pretty good snowfall each year so it makes sense that something like this would form. I haven't really been on a spelunking trip or anything like that. I've visited a few caves before, but nothing quite on this level. Looks like a cool first experience.