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.
A stunning photograph of a submerged tree taken by Flickr user Melinda while visiting the incredibly beautiful Jiuzhaigou Valley in China. She also runs a great blog with all kinds of beautiful landscape goodness, well worth a visit.