Mount Rainier in Washington has recently sprung to life, exhibiting nearly continuous activity over the past few days, raising concerns about a potential eruption in the near future.

This volcano is among the most hazardous in the United States, looming over millions of residents in key cities such as Seattle, Tacoma, and Yakima in Washington, not to mention Portland, Oregon’s most populated city.

Since Saturday, Mount Rainier has been experiencing a series of persistent tremors beneath its surface, best characterized as a multitude of small vibrations merging together.

The Pacific Northwest Seismic Network (PNSN) has detected these ongoing seismic disturbances, noting that seismometers on Mount Rainier have registered three consecutive days of almost uninterrupted, high-energy seismic signals on the volcano’s western flank.

Unlike seismic events associated with large earthquakes, the patterns observed in Washington resemble a volcanic tremor—a continuous hum or roar generated by the movement of magma, hot water, and gases within the volcano.

It doesn’t mean Mount Rainier is going to erupt at any moment, but it is a warning sign that volcanic activity could eventually build towards a critical level.

Geologists will be watching to key signs of this volcanic tremor escalating, including its severity increasing in the coming days, actual earthquakes starting inside the volcano, and the ground at Mount Rainier beginning to swell.

When this volcano eventually explodes, it won’t be scorching lava flows or choking clouds of ash that threaten Americans, but the lahars: violent, fast-moving mudflows that can tear across entire communities in mere minutes.

Mount Rainier, one of the most dangerous volcanos in the US, looms over Olympia, Washington. This city is home to more than 50,000 people

The activity at Mount Rainier started with a sharp spike around 5am ET on November 15. After that, the line gets fuzzier and fuzzier, showing vibrations that never calms down

Large lahars can crush, bury, or carry away almost anything in their paths, according to the US Geological Survey.

Volcano watchers on social media spotted the fresh readings from PNSN, noting that vibrations under the mountain went from normal to chaotic in a matter of hours.

Seismometers registered a sharp spike around 5am ET on Saturday, which proceeded to grow in strength throughout the day.

On seismic instruments, this was recorded as a line that continued to get fuzzier and fuzzier as the more time passed, eventually looking like a thick black line as the vibrations increased.

By November 16, almost the entire 24 hours lit up seismometers with a solid thick black band with very little quiet time.

On Monday, the readings at Mount Rainier showed an almost solid black line, revealing overlapping vibrations for 80 to 90 percent of the day.

None of the next warning signs for an eruption have emerged on Tuesday, so the threat level has not been raised by USGS.

However, this volcanic tremor has been one of the longest episodes at Rainier in recent years, and USGS continues to list the overall risk of a future eruption as ‘Very High.’

The volcanic tremor continued into November 16, with seismometers recording almost no quiet moments throughout the entire day

On Monday, the readings from Pacific Northwest Seismic Network recorded an almost solid map of activity on Mount Rainier’s western slope

Mount Rainier has already experienced massive swarms of earthquakes earlier this year, sparking fears of an imminent volcanic explosion.

In July, over 1,000 earthquakes rocked the area over a span of more than three weeks, in the largest ever seismic swarm recorded at the mountain.

That swarm easily surpassed the last large string of earthquakes at Mount Rainier, which came in 2009, lasting only three days and producing around 120 minor earthquakes.

The swarm in July started during the morning of July 8 saw up to 41 minor earthquakes registering every hour throughout the rest of the month.