It’s been 36 years almost to the day when Moutn St. Helens in Washington state erupted, and rumblings beneath its surface have experts worried it may be ready to blow again soon.

Scientists have discovered a swarm of miniature earthquakes under the mountain ever since March, suggesting that the volcano is recharging with magma, according to a Q13Fox.com report.

The U.S. Geological Survey has detected more than 130 earthquakes between 1.2 and 4 miles deep since March 14. The earthquakes only register about a 0.5 on the Richter scale, and many more quakes have probably happened that are too small to detect.

This doesn’t necessarily mean that Mount St. Helens will erupt very soon. Magma recharges can happen for years beneath a volcano without an eruption, and so far no shallow seismicity or ground inflation has been observed at Mount St. Helens.

Mount St. Helens is located in the Cascade Mountains, about 55 miles northeast of Portland and 95 miles south of Seattle. There have been four major explosive eruptions in the last 500 years, including the 1980 eruption, which was the deadliest and most economically destruction volcanic event in U.S. history. The eruption killed 57 people and demolished 250 homes. The debris avalanche resulted in an earthquake measuring 5.1 on the Richter scale.

The USGS has closely monitored the volcano since then, and scientists frequently study it to learn more about volcanoes.

“On March 16, 1980, the first sign of activity at Mount St. Helens occurred as a series of small earthquakes,” the USGS said in a statement on the 1980 eruption. “On March 27, after hundreds of additional earthquakes, the volcano produced its first eruption in over 100 years. Steam explosions blasted a 60- to 75-m (200- to 250-ft) wide crater through the volcano’s summit ice cap and covered the snow-clad southeast sector with dark ash.

“Within a week the crater had grown to about 400 m (1,300 ft) in diameter and two giant crack systems crossed the entire summit area,” it continued. “From the start of the eruption, the bulge grew outward—nearly horizontally—at consistent rates of about 2 m (6.5 ft) per day. Such dramatic deformation of the volcano was strong evidence that molten rock (magma) had risen high into the volcano. In fact, beneath the surficial bulge was a cryptodome that had intruded into the volcano’s edifice, but had yet to erupt on the surface.”