The first time I watched this I was thinking, that doesn't look like the inside of a cave, where are the stalamites? Then I noticed the rings where towers "might" have been, and remembered that most cave features are extremely fragile, and simple rainfall would probably dissolve them down quickly; it made sense after that.
Cool time capsule seeing how geography can build something up over millenia, it collapses, and biology takes it over in short order.