North Korea is at risk of a massive explosion — and not from its infamous nuclear tests.

A massive active volcano on the border of North Korea and China known as Mount Paektu is starting to rumble, and the new tremors are causing international scientific interest in the mountain, which caused one of the most violent eruptions in human history a thousand years ago, according to a Christian Science Monitor report.

Now, a team of scientists from China and North Korea as well as those from the West have deployed six seismometers on the North Korean side of the volcano. Because of North Korea’s reclusive nature, they hadn’t been able to do that before, which prevented them from getting a complete picture of the volcano.

The eruption in approximately 946 AD has been called the “Millenium Eruption,” and it sent debris as far as Japan.

It rumbled a bit in 2002 and 2005, prompting renewed interest. More recent tremors suggest that scientists should really be studying this beast more closely, and the fact that North Korean officials invited outsiders in shows that they’re pretty concerned about it.

“Despite being responsible for one of the largest eruptions in history, comparatively little is known about its magmatic evolution, geochronology, or underlying structure,” an abstract from the paper on Mount Paektu reads. “We present receiver function results from an unprecedented seismic deployment in the DPRK. These are the first estimates of the crustal structure on the DPRK side of the volcano and, indeed, for anywhere beneath the DPRK.

“A large region of the crust has been modified by magmatism associated with the volcanism,” the abstract continues. “Such high values of VP/VS suggest that partial melt is present in the crust beneath Mt. Paektu. This region of melt represents a potential source for magmas erupted in the last few thousand years and may be associated with an episode of volcanic unrest observed between 2002 and 2005.”