Fan and Mortar Geysers, Geothermal formations in Upper Geyser Basin, Yellowstone National Park, United States.

Fan and Mortar Geysers are two cone geysers on the same bank of the Firehole River in the Upper Geyser Basin. Mortar is the large mortar-shaped cone on the left, with Upper Mortar, Lower Mortar and a smaller vent. Fan has at least seven vents aligned roughly perpendicular to the river, from River Vent to East Vent. Spiteful Geyser sits close to the boardwalk nearby.

Until 1925 Fan and Mortar were two close but independent geysers, far weaker than today; sometime before that year they connected at depth and have since erupted as one large system. Fan was named by the Washburn–Langford–Doane Expedition of 1870, and Frank Jay Haynes named Mortar in 1883 for eruptions resembling mortar discharges.

The names record early park exploration: Fan was named by the Washburn–Langford–Doane Expedition of 1870, and Mortar by photographer Frank Jay Haynes in 1883, who thought its eruptions resembled mortar fire. Visitors today watch the sequence of vents—River, High, Gold, Angle—before a possible eruption.

You can view both geysers from a bridge and boardwalk that leads to a larger thermal pool area. Wear sturdy shoes since the ground is uneven and can be slippery from mineral deposits and moisture.

Beneath the ground, the two geysers are connected through an underground channel system that sometimes causes them to erupt in sync. This subsurface link makes their eruption patterns unpredictable and means no two visits display the same behavior.