"Natural caves" refers to underground hollow spaces that are formed by natural processes such as erosion or volcanic activity, rather than being human-made. Full definition
We live in an area with a lot of natural caves and a lot of opportunities to learn about bats and how they influence our environment. (wellnessmama.com)
When the lava stream clears, you're left with an incredible natural cave where you can walk the path of a volcano. (blog.queensland.com)
On this tour, trained guides lead visitors through a living natural cave system to witness the ongoing miracles that created this masterpiece of underground wonders. (anywhere.com)