In the 19th century, Excelsior Geyser was the largest geyser in the world; over 200 feet wide and 300 feet long, it erupted up to 300 feet in the air -- it must have been quite impressive to see. Sometime in the late 1880s, Excelsior went dormant, though it oddly sprang to life in 1985, for just a couple of days, erupting to a much lesser height.

Excelsior is now a huge hot spring which produces a tremendous flow of hot water: over 4,000 gallons per minute! The water percolates up from the super-hot main crater (visible as a deep blue area on the right in this image) and then flows downwards through a series of channels, ending up in the Firehole River. The main channel flowing down to the river has such a high flow that it is practically a river itself! The large size of the spring has led to the creation of enormous yellow, orange and red microbial mats, seen here on the left.