Additional Information: Black Sand Basin, an isolated group of the Upper Geyser Basin, was originally named the Emerald Group by A. C. Peale in 1878. But turn of the century tourists began calling it Black Sand Basin because of the small fragments of black obsidian sand which cover portions of the basin. Black Sand Basin contains a small collection of jewel-like geysers, and colorful hot springs. Emerald Pool is the most colorful and famous of these springs. It is a deep emerald green fringed by an outer ring of yellow and orange. Another colorful pool is Opalescent Pool. This recently formed pool inundated a stand of lodgepole pine, creating a stand of white skeletons amidst a rainbow-colored pool. An unusual geyser formed on the bank of Iron Creek. Cliff Geyser formed a rim or wall-like ridge of sinter around its crater from which it erupts 30 to 40 feet high. The famous Handkerchief Pool was once the drawing attraction to Black Sand Basin. Turn-of-the-century tourists dropped their handkerchiefs into this small spring. Convection currents then whisked their laundry away where it would reappear again at the surface, freshly laundered. RAINBOW POOL Temperature 161°F Dimensions 100x130 feet. Depth 27 feet. The edges of this pool display the color of the rainbow, hence the name. Algae and cyanobacteria are responsible for the varied colors. This pool has only erupted a few times in the past. During one eruption in 1948 it reached a height of 25 feet. The last known eruption was in 1973. The famous Handkerchief Pool, located along the southern edge of Rainbow, was a popular pool at the turn-of-the-century. Tourists dropped handkerchiefs at one end of the pool and convection currents would pull them under. A moment later they would reappear in another vent, freshly laundered. The pool has not functioned since 1929 when it became plugged by human vandalism. It is now a small spouter and inaccessible because of microbial mats. Underground connections exist between Rainbow Pool, Green Spring and Handkerchief Pool
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