Here's an interesting article on natural pools. Has anyone ever done or seen something like this?
http://www.usatoday.com/news/nation/story/2012-09-11/natural-swimming-pools/57750782/1?csp=hf
http://www.usatoday.com/news/nation/story/2012-09-11/natural-swimming-pools/57750782/1?csp=hf