Today the volcano that straddles the border between China and North Korea is tranquil. Hot springs simmer on the surrounding peaks, wild blueberries grow on its green slopes and a crystal-clear pool called Heaven Lake fills its crater.

But Mount Paektu, as the North Koreans call it, is only asleep. When it last awoke about a thousand years ago, the so-called Millennium Eruption unleashed one of the most violent volcanic events in recorded human history. And when North Korean scientists recorded a swarm of tiny earthquakes rumbling beneath the volcano from 2002 to 2005, they were so concerned that the reclusive country eventually contacted the West for help.

The result was a rare collaboration of scientists from North Korea and researchers from countries with which it has hostile relations.

Officials in Pyongyang first reached out in 2011, and after two years of planning the project was set. In 2013, volcanologists from the United States and Britain met researchers in North Korea to investigate Mount Paektu and its magma plumbing.