Sawmill Geyser Complex

The Sawmill Complex (also known as Sawmill Group, although Sawmill Group also refers to a larger collection of features on the North side of the Firehole River, across from the Castle Group) is a collection of interconnected geysers found in the Upper Geyser Basin. This complex is very active, though cyclic - while it almost always contains active geysers, they are rarely active at once, sharing a complicated plumbing system.

Geysers in this Group
* There is some debate about whether Bulger is a part of the Sawmill Complex or the Grand Geyser Complex; at any rate, the geysers and springs in this area are highly interconnected and ties between the two groups are complicated.
 * Sawmill
 * Penta
 * Churn
 * Spasmodic
 * Tardy
 * Old Tardy
 * Bulger Spring *
 * Uncertain
 * Nifty
 * Slurp
 * Oval Spring
 * Twilight Spring

Activity
This complex has two main eruption modes, the first and most historically common of which is "Sawmill Mode" in which Sawmill, Spasmodic, and Tardy geysers are most active. Alternatively, the complex can be in "Penta-Churn" mode, in which Sawmill is dormant, even when Spasmodic and Tardy experience activity.

A third, rarer eruption mode is known as "Spasmodic" mode in which the only significant eruptions occur in Spasmodic Geyser while the other features remain below overflow levels. This mode appears to be caught between a "Sawmill" and a "Penta-Churn" mode, and can last months to years. This mode occurred from January 2017 to at least May 2018.

As of August 2019, ___. Sawmill, Tardy, and Penta geyser had not erupted since 2017, and Uncertain since 2016. However, Old Tardy, Churn, and Spasmodic were quite active.