Back in the summer of 2016, a big earthquake struck northwestern Turkey. That’s not so unusual, considering that the region sits atop a highly active branching fault network that has a history of producing some seriously powerful tremblors.

The strange thing about this particular quake is that it lasted for 50 days, and not a single soul felt it.

According to a new study in Earth and Planetary Science Letters, the temblor was a very peculiar type of earthquake known as a slow slip event. Unlike “typical” earthquakes, which crack the crust with a sudden jolt, slow slips involve very gradual movement along a fault. They release none of the damaging seismic waves you might normally expect, which means