The Greek island has a long history of earthquakes, but what is causing the recent series of quakes has scientists perplexed.

Millions of people around the world live near one of the roughly 1,500 active volcanoes on Earth—from Italy’s Campi Flegrei to Indonesia’s Merapi to the United States’ Mount Rainier.

Despite being more than 600 kilometres from the nearest volcano, the river averaged 86C. According to National Geographic, that is hot enough “to cook a small animal in seconds.” What makes it ...