A new study proposes a slightly scary idea: what if “extinct” volcanoes aren’t actually extinct, but just waiting? Slowly building a magma stockpile over time so they can one day reawaken and explode with the fury they haven’t exhibited in centuries?

That’s the conclusion of volcanologists at ETH Zurich in Switzerland, who published their findings in Science Advances, after examining the Methana volcano in Greece. Scientists reconstructed roughly 700,000 years of its history using zircon crystals, microscopic minerals that form in magma chambers and act as little timestamps of volcanic history. They suspect that just because a volcano has been quiet for decades or even centuries, it doesn’t mean it’s dead. It might just be gathering his strength to pop off again later.

Methana appeared dormant for over 100,000 years. That’s more than enough time to label a volcano “extinct.” But beneath the surface, during this quiet period, zircon production actually peaked, signaling that magma was still forming and accumulating underground. It wasn’t dead, and it wasn’t even necessarily asleep. It was still bubbling deep beneath the surface.

They found that deep underground, high pressure keeps magma molten. As it rises, pressure drops, water escapes like gas from a shaken soda bottle, and the magma thickens. Eventually, it stalls before reaching the surface. All this combines to pause eruptions, but the system is far from dead as it continues “’breathe’ underground for millennia without ever breaking the surface,” according to the study’s senior author, Olivier Bachman, in a press release.

This behavior helps explain why so many volcanoes go quiet for tens of thousands of years before finally erupting again. The finding changes how researchers have to assess a volcano’s risk. Current models are based on the last time a volcano arrived. But the new research is saying that a lack of recent activity is potentially dangerously misleading.

If dormant volcanoes can slowly recharge over centuries, hazard monitoring systems need to adapt. Volcanologists need to use every tool at their disposal to reveal which supposedly dead volcanoes have been steadily preparing for a major comeback for a thousand years, and in that time, cities and villages arose around them as what could one day truly be considered outdated science led its citizens to believe that this once mighty and destructive volcano has since been reduced to a big, pretty mountain. But it hasn’t. It is still as potentially destructive as ever; it’s just been quiet about it.