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.

Over 200 earthquakes have been detected around Mount Rainier, Washington, raising eyebrows during the biggest swarm of tremors in nearly two decades.

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