The Great Prism (also known as the Great Rainbow) is the largest hot spring in the United States and the third largest hot spring in the world. It is named "the most beautiful surface of the earth". In 1871, it was discovered by geologists and named the Great Prism for its striking color. Hot spring water is rich in minerals, so that algae and colony of coloured bacteria can survive in the water, thus showing these colours. Hot springs show blue, green, yellow, orange, orange and red colours from inside to outside, while hot springs have no living things in the center because of the high temperature. A wooden walkway was built around the prism and it took about 40 minutes to walk down in a circle. The best angle to see the big prism is overlooking, but when we drove to view point, we found that the road was blocked, and the sign said that because of the presence of bears, visitors were not allowed to go up the hillside viewing platform. Unfortunately. Plus when we were on the Great Prism Trestle, the fog was so big that we could hardly see it, so the scenery was only four stars.