A major volcano in Nicaragua has not one but two active magma chambers, scientists reveal in a new study, shedding important light on the inner workings of a highly active volcano located near a large human population.

Masaya in western Nicaragua is one of the region’s busiest volcanoes, although its activity in modern times has been mostly “passive degassing, lava lake formation, and vent-clearing explosions,” the study’s authors write in Geophysical Research Letters.

It’s a basaltic shield volcano, so that behavior isn’t surprising. Shield volcanoes are known for highly fluid lava that erupts in slow, effusive flows – such as typical Hawaiian eruptions – rather than the violent blasts of stratovolcanoes.