One of Yellowstone National Park’s most photographed sites, Grand Prismatic Spring, displays a remarkable rainbow of colors that comes from thermophiles – microbes that thrive in scalding water. It has the distinction of being the park’s largest hot spring. It measures approximately 370 feet (112.8 m) in diameter and is over 121 feet (37 m) deep. A description of this spring by fur trapper Osborne Russell in 1839 also makes it the earliest described thermal feature in Yellowstone that is definitively identifiable.Yellowstone contains the largest collection of hydrothermal features on Earth. The source of this heat is a plume of magma that extends more than 600 miles below the park’s springs and geysers. Scientists suspect this plume may descend as far as 1,800 miles down, all the way to Earth’s outer core-mantle boundary.Grand Prismatic Spring is one of the many attractions that brings over four million people to Yellowstone each year. This is up from 13,727 in 1904 when the park first counted visitors.