In Jiuzhaigou Valley, Sichuan providence, China there is a lake that is a clear and clean as can be. It is known for its many multi-level waterfalls and colorful lakes, and was declared a UNESCO World Heritage Site and holds 40% of all plant species found in China.
An underwater world Grüner See (Green Lake) several months of the year in Austria when the snow melts. You can see a clear view of the bottom because of the freshwater from the snow. It's like a fishtank! Terrestrial flora cannot grow at the bottom because of the rocks and water density.