Formed in Carboniferous/Permian limestone: also known as 47 million to 358 million years. Okay, this certainly wasn't in it's present state 20 million years ago. However my point is it is a very old and slow growing structure. Obviously a Doline is evidence of instability at one point in time. But when a structure exists for millions of years. Let me do the math here: 2 Dolines over 20 million years, screw it, I don't have to do the math. I'm comfortable walking through that structure.