Hamilton Pool is a swimming hole west of Austin, TX by about 30 miles or so. From what I gather, it was an underground lake or river in which the roof of the cave that it was in fell in and left it looking like so. Pretty cool place I must say.
This shot made it as an Editor's Pick on the HDRSpotting website. It's also pretty popular there as it's hovering around #9 or #10 overall. HDRSpotting is currently having a contest in November for the most popular HDR. If you win, you get $100. Check it out if you have time. There are some great HDRs there and you can see this Hamilton Pool shot on HDRSpotting here.