Earthquakes have been rumbling more frequently deep beneath Mauna Loa (search), suggesting that the world's largest volcano is getting ready to erupt for the first time in 20 years, scientists said.

"We don't believe an eruption is right around the corner, but every day that goes by is one day closer to that event," said Paul Okubo, a seismologist at the Hawaiian Volcano Observatory (search) on the Big Island.

Mauna Loa erupted for three weeks in 1984, sending a 16-mile lava flow toward Hilo. Since then, the U.S. Geological Survey (search) estimates that more than $2.3 billion has been invested in new construction along Mauna Loa's slopes.

Since July, more than 350 earthquakes have been recorded far beneath the 13,677-foot-high Mauna Loa, said Don Swanson, scientist-in-charge at the observatory.

"Mauna Loa is grumbling, growling and getting ready to come out of its den," he told West Hawaii Today for Sunday's editions.

The earthquakes have been what seismologists call "long period," which means their signals gradually rise above the noise generated by usual seismic activity.

"Such a concentrated number of deep, long-period earthquakes from this part of Mauna Loa is unprecedented, at least in our modern earthquake catalog dating back to the 1960s," Okubo said.

While forecasting an eruption cannot be exact, Okubo noted that the mountain today is wired with more state-of-the-art tracking and measuring technology than ever before.

The definite sign of an impending eruption is an earthquake swarm — a dramatic increase in the number of daily tremors from a handful, to dozens to ultimately hundreds, Okubo said.

Mauna Loa is within Hawaii Volcanoes National Park (search), which also contains the well-known Kilauea volcano. Kilauea has been erupting continuously since Jan. 3, 1983.