Grotto is perhaps the most unusual of Yellowstone’s geysers. Geologists believe that hundreds or thousands of years ago, Grotto Geyser emerged in a stand of dead or dying trees and, through time, deposited layer upon layer of siliceous sinter, or silica, over the stumps and branches. Its initial eruption can reach heights of more than 40 feet, but then generally subsides to about 15 feet. The duration of its eruption can vary from about 50 minutes to more than 24 hours.