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.

A swarm of hundreds of small earthquakes have been detected rumbling deep below Mt. Rainier in Washington State early Tuesday morning, though geologists stress there is no cause for concern.

Hundreds of small earthquakes have rattled Mount Rainier this week in the largest swarm of tremors in more than a decade.