For decades, a mysterious swarm of earthquakes has rumbled beneath the small town of Pahala near the southern coast of the island of Hawaii. By 2015, the rate of subterranean trembles had ticked up from about seven to 34 quakes per week. And the year after Kilauea's 2018 eruption—the largest Hawaii has seen in centuries—the quakes reached a feverish pitch.

Nearly 500 earthquakes shook underneath Pahala every week, and the heightened activity hasn't let up. "We're like earthquake central down here," says Lou Daniele, general manager at Ka'u Coffee Mill in Pahala. "It's just become a constant part of daily life."

Now scientists have discovered the source of this geologic ruckus: a stack of interconnected features some 22