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.

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