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.

Mount Rainier, the active volcano towering above southwestern Washington state, started rumbling — very lightly — on Tuesday. A swarm of small earthquakes was detected under the mountain triggering focused monitoring from officials. But researchers have determined there is no current threat of an eruption.