A volcano erupted without warning. Now, scientists know why.

Last year, one of the most dangerous volcanoes in Africa erupted without warning.

In a way, Nyiragongo, a vertiginous volcano in Congo, is always erupting: The mountain is crowned by a rare, persistent lave lake constantly fed by churning magma below. But on May 22, 2021, its molten innards found another route to the surface. They oozed from fractures on the volcano’s flanks toward the metropolis of Goma, leading to the deaths of at least 31 people, injuring 750 others, displacing thousands more and leaving behind a trail of destruction.

Now, in a new study published Wednesday in Nature, Delphine Smittarello, a geophysicist at the European Center for Geodynamics and Seismology in Walferdange, Luxembourg, and her colleagues articulated how the eruption managed to ambush everyone.

Most sufficiently monitored volcanoes offer warning signals before erupting. Magma forcing its way through rock generates distinctive types of earthquakes, deforms the land as it ascends and unleashes noxious gases. Some volcanoes are so active that they are always creating noticeable chaos, but a distinct change in their usual, or “background,” behavior betrays their eruptive ambitions.