Below the multi-hued pools and gurgling geyers of the United States’ Yellowstone National Park resides a geologic hot spot that has driven some of the biggest eruptions on the planet Earth. Now having thoroughly mapped the area, geoscientists have identified not one but two subterranean magma chambers.

Scientists had already known of a magma chamber approximately 10 kilometers down, with a molten material volume of 10,000 cubic kilometers. But a deeper and much larger one has now been discovered, about 4.5 times larger. This one is 20 to 50 kilometers below the surface and is “the missing link” the more shallow mantle chamber already known about and the mantle plume, a channel that brings molten material to the surface.

The last time the area saw a major eruption was 640,000 years ago and the chances of another have not increased with the discovery, which would be driven by an emptying of the shallow chamber. However, what the newly-identified, deeper chamber means is that the eruption-inducing shallow chamber can be refilled over and over. The discovery was reported in Science. A video from the University of Utah shows a three-dimensional rendering of the pools.

The subterranean mapping was done with seismometers, which measure sound. When seismic waves pass through liquid, they get slower and where this happens the area is interpreted as a magma chamber. Although such areas consist mostly of solid rock, the small amount of liquid can nevertheless be detected.

Eleven of the 400 seismometers that make up the USArray of Earthscope were used while another 69 gathered data from a number of local seismic networks.