@judello and I swam in Jacob's Well, also known as "The Blue Hole", near Wimberley, in the Hill Country of Central Texas.
There appears to be no bottom and the feeling is uncanny. There is a video taken by a diver that shows just how confusing it becomes, the deeper you descend. Dangerous? Hell, yes! People have drowned in Jacob's Well, some of them experienced divers.
Photo by Woody Welch