Mats of algae, bacteria, and other microscopic organisms color the edge of Grand Prismatic Spring in Yellowstone National Park in shades of orange, yellow, and red. Scientists first discovered organisms living in extremely hot waters in Yellowstone in 1967. Other organisms have since been found in other extreme environments, such as super-heated water around vents on the ocean floor, suggesting that life is tenacious and could be found in difficult environments on other worlds. [Jim Peaco/National Parks Service]