Jason de Caires Taylor’s underwater sculpture garden in Cancun. Constructed out of concrete and steel, and bolted to the ocean substrate, the works here act as artificial reefs that provide “an ideal habitat for filter feeding organisms.” http://www.dedeceblog.com/2010/10/13/cancun/
Austria's Green Lake in the Hochschwab Mountains is a hiking trail in the Winter. The snow melts in early Summer and creates a completely clear lake. The Lake has a grassy bottom, complete with underwater trails, park benches and bridges. So cool!