What stinks in Yellowstone? (Yellowstone monthly update – September 2026)

When you get out of your car at Mud Volcano, your nose immediately detects something that smells like rotten eggs. That smell is hydrogen sulfide, H2S. It’s very common in geothermal systems like the ones in Yellowstone.

Where does it come from? Deep beneath the surface, gases are dissolved in magma. One of the gases that’s trying to escape is sulfur dioxide (SO2). As SO2 rises away from the magma chamber, it encounters water and a chemical reaction takes place changing the sulfur dioxide into hydrogen sulfide. It’s that stinky H2S that our noses detect at the surface.

One of the reasons we monitor gas emissions at volcanoes is to track SO2. A lot of SO2 might indicate magma is on the rise or that there is a clear pathway between magma and surface with no water in the way. So, the fact that we can sense H2S in Yellowstone is a good thing!  It means that the volcanic system remains quiet.

During the month of September 2026, the University of Utah Seismograph Stations, which monitors and operates the Yellowstone seismic network, located 77 earthquakes. The largest was a magnitude 3.4. There was one seismic swarm during the month, with about 24 events northwest of West Yellowstone, near Hebgen Lake.

There has been no significant uplift or subsidence of the caldera since early 2026.

Echinus Geyser erupted on September 15. Giant Geyser had two eruptions on September 10 and 22. Black Diamond Pool, which was the source of a large hydrothermal explosion in 2024, had several eruptions recorded by seismic, acoustic, and camera monitoring stations. A small hydrothermal explosion occurred on September 27 from the collapse pool that formed in Biscuit Basin in June.

Yellowstone volcano remains at normal, background levels of activity.

For questions, email yvowebteam@usgs.gov

Read Caldera Chronicles https://www.usgs.gov/volcanoes/yellowstone/caldera-chronicles

Visit Yellowstone Volcano Observatory website https://www.usgs.gov/volcanoes/yellowstone