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.

Hundreds of earthquakes are detected at or near Mount Rainier each year, though typical swarms are smaller than what started last night.

A “swarm” of hundreds of small earthquakes are continuing to hit Mount Rainier this week. The U.S. Geological Survey Cascades Volcano Observatory and the Pacific Northwest Seismic Network said Tuesday that the shaking began around 1:30 a.m. At times, several quakes occurred per minute, ranging from depths of 1.2 to 3.7 miles below the summit.