The Grand Prismatic Spring in Yellowstone National Park, US, is known for its striking coloration and enormous size. The vivid colors in the spring are caused by bacteria, living on the edges of the mineral rich water. The water in the center of the pool looks extremely clear, and has a beautiful, deep-blue color – thanks to the scattering of blue wavelengths – because the water is so hot that’s actually sterile.
By Jim Peaco, National Park Service – http://www.nps.gov/features/yell/slidefile/thermalfeatures/hotspringsterraces/midwaylower/Images/17708.jpg transferred from the English Wikipedia, original upload 1 April 2004 by ChrisO, Public Domain, Link
By Yellowstone National Park from Yellowstone NP, USA (Grand Prismatic Spring) [Public domain], via Wikimedia Commons