Grand Prismatic Spring is the largest hot spring in Yellowstone. The colors around shore are bacteria and algae that tolerate 170 degree F temps. There is a boardwalk along GPS but it hard to visualize the colors at ground level so I found a trail from the Fairy Falls parking lot - it is an easy walk - about 15 minutes from the trailhead. There are some trails cutting up hill to informal viewpoints - it is well worth the climb - what an amazing sight!
I took this photo on a gorgeous day in Yellowstone National Park in May 2009. Absolutely beautiful morning as I headed from West Yellowstone SE through various geyser basins, including the Lower, Midway, and Upper Basin. Each one was different but all were worth stopping at. Enjoy the photos!