Mount Rainier was experiencing "the most significant seismic activity" at the volcano in more than 15 years as an "earthquake swarm" hit the site starting on Tuesday.

On Tuesday, hundreds of earthquakes were detected, with depths ranging from 1.2 to 3.7 miles below the summit.

Swarms — or clusters of earthquakes that occur in the same area in quick succession — are relatively common on Mount Rainier.