The monthslong eruption of Hawaii’s Kilauea volcano in the spring and summer of 2018 was the most destructive of its recorded history. As the summit crater periodically collapsed, 320,000 Olympic-sized swimming pools’ worth of lava was squeezed out of fissures on its eastern flank. Flaming rivers of molten rock destroyed 700 homes while building new volcanic shorelines, vaporizing lakes and conjuring bizarre weather.

What kick-started such a dramatic sequence? A study published Wednesday in Nature suggests an unusual suspect: rainfall.

In the months before the eruption, Hawaii was inundated by above-average precipitation. This rainfall would have found its way into the pores present within Kilauea’s volcanic rocks. If too much water tries to cram into these pores, the rock fragments from the inside out.

This new model suggests that, just before the outburst of lava in May 2018, the rocks surrounding the volcano’s cache of magma were experiencing a 47-year peak in pore pressure — enough to break down the walls holding its magma in place until the liquefied rock made a break for it.