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.

The U.S. Geological Survey said it and other agencies were monitoring the situation and would issue additional alerts "as needed."