Unofficially-named "A-0" Geyser lies about a hundred feet southeast of the Surprise Pool parking area along the Firehole Lake Drive. You can view the geyser from the parking area; however, contrary to older descriptions, you CANNOT view it from the former trail that headed southeast from the Great Fountain Geyser parking area. This trail has been closed by the Park due to overuse and abuse, and using it is now illegal. Please don't get yourself into trouble by following out-of-date information.
The name "A-0" comes from the geyser's proximity to A-1 and A-2 geysers, though its appearance is much different from the currently-inactive other two geysers. "A-0" can be identified by its well-defined, but shallow, round basin. In the center of the basin is a round vent, giving the geyser the appearance of a circle within a circle. No pool forms as water bursts from the vent to heights between 5 and 10 feet. During the 1999 Spring/Summer season intervals averaged around 24 minutes with durations averaging around 30 to 40 seconds. There is another very small, unnamed geyser between "A-0" and Botryoidal Spring which erupts about every 10 minutes from several small holes to a height of a few inches to two feet.