Black Opal Pool
Located in the Midway Geyser Basin of Yellowstone National Park, the Black Opal Pool hot spring usually has a temperature of approximately 132 °F (56 °C). Though generally active as a hot spring, Opal Pool is considered a fountain-type geyser. The first recorded eruption of Opal Pool was in 1947, recurring in 1949, 1952, and 1953, then ceasing. Eruptions resumed in 1979, happening at least once in most following years. Eruption heights are typically under 30 feet (9.1 m) in size, but some eruptions are seen with peaks of 70 feet (21 m) to 80 feet (24 m). Eruptions occur suddenly following visible convection in the pool but are unpredictable. An outbreak consists of one giant burst that throws water 20–80 feet high, making Opal Pool the largest active geyser at Midway Geyser Basin. Much smaller splashes seconds apart stretch the total duration to about 1 minute. In 2005, Opal wholly drained but refilled as a vivid green pool in 2008.