The temperature of Opal Terrace is 160°F. The feature was named in 1871 by the Hayden Expedition when it was a small spring with opal colors. It became dormant for several years until 1926 when it began to flow again. During the 1940s Opal Terrace was very active and rapidly deposited travertine, doubling its size. Opal is still known for its pastel colors, but like most Mammoth Terrace features it constantly changes activity, shape and color.