This was one of my favorites (and aptly named). It's one of the deepest pools in the park at 53 feet. The green color comes from the blue water mixing with yellow microorganisms. This used to be an active geyser, then lay dormant for eighty years. In the early 1990's it erupted multiple times (up to 100 feet) and drained some of the nearby pools. I can't imagine standing next to one of these "dormant" features only to be startled by an eruption!