Echinus (e-KI-nus) Geyser was a perennial crowd-pleaser which typically erupted every 35 to 75 minutes. Late in 1998, this geyser altered its interval and now erupts only a few times per day at best. Its pool fills gradually with water; then suddenly, bursts of steam and water explode 40 to 60 feet (12 to 18 m) skyward. Eruptions usually last about 4 minutes, but in the past major eruptions have lasted as long as 118 minutes. The major eruptions were believed to be caused by a secondary water source which has mysteriously vanished. There has not been a major eruption in 3 years. In late 1998 Echinus' performance diminished and became erratic. As of the summer of 2007, its eruptions continue to be far less predictable than they were in the 1990s.
Echinus is the largest acid-water geyser known. Its waters are almost as acidic as vinegar with a pH ranging from 3.3 to 3.6. Acid geysers are extremely rare with the majority of the planet's total being found here at Norris Geyser Basin.