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An underwater volcano off the coast of Oregon has risen from its slumber and may be spewing out lava about a mile beneath the sea.

Researchers were alerted to the possible submarine eruption of the Axial Seamount, located about 300 miles (480 kilometers) off the West Coast, by large changes in the seafloor elevation and an increase in the number of tiny earthquakes on April 24.

Geologists Bill Chadwick, of the Pacific Marine Environmental Laboratory and Oregon State University, and Scott Nooner, of the University of North Carolina, Wilmington, successfully forecast the eruption in a blog post in September 2014, though they had presented their ideas at a meeting before then. [Axial Seamount: Images of an Erupting Undersea Volcano]

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Axial Seamount is an underwater mountain that juts up 3,000 feet (900 meters) from the ocean floor, and is part of a string of volcanoes that straddle the Juan de Fuca Ridge, a tectonic-plate boundary where the seafloor is spreading apart.

Axial Seamount, an undersea volcano located 300 miles (480 kilometers) off the coast of Oregon, appears to be erupting. Bill Chadwick, Oregon State University

Chadwick and Nooner have been monitoring the seamount for the past 15 years by measuring tiny movements in the seafloor as the volcano inflates with magma and then deflates. During that period, the volcano has erupted two other times — once in 1998 and again in 2011.

"It's kind of like a balloon — as magma is going into the balloon, it's inflating, and it pushes the seafloor up," Chadwick told Live Science. "As more and more magma gets in, the pressure builds. Eventually, it reaches some critical pressure where [the seamount] can't hold it in anymore, and then it squirts out."

After the volcano erupts, the seafloor drops very rapidly, "like letting air out of a balloon," he said.

For the first time, Chadwick and his colleagues were able to observe the eruption in real time, thanks to a set of instruments connected to shore by a fiber-optic cable, installed last summer by the University of Washington and paid for by the National Science Foundation.