Sleeping giant stirs. (Freeze Frame).

Spectacular Kilauea (KEE-lah-WAY-uh) may be the volcano that won't quit. It's been erupting nonstop for 19 years oil the island of Hawaii, ejecting streams of lava (erupted molten rock) at a searing 1,100[degrees]C (2,012[degrees]F) into the Pacific Ocean. But scientists grew wary last May when a powerful new jet of lava burst from a narrow vent on the volcano's flank. Hours later, the surface of neighboring Mauna Loa--the world's biggest volcano--began swelling with molten rock after two decades of inactivity. "There appears to be some connection between the two," says Stanford University geophysicist Paul Segall, who has been studying Hawaiian volcanoes since 1990.

While Kilauea's lava usually flows harmlessly into the ocean, Mauna Loa's eruption could devastate nearby neighborhoods. Since the volcano's last eruption in 1984, $2.3 billion worth of new construction has been built on the volcano's slopes, which sprawl over half the island. Is an explosion imminent? Scientists don't think so: At this point, "Magma [molten rock] is just flowing up from deep within Earth into a large reservoir located three kilometers below the volcano's summit," Segall says.

Nearly 95 percent of the world's volcanoes form near tectonic plate boundaries, where Earth's continents converge. But the seven volcanic islands that compose Hawaii formed over what geologists call a hotspot, a huge plume of magma that oozes from Earth's mantle to the crust. "It punches through the plate every million years or so, creating a new volcano." Segall says.

Experts don't know if Mauna Loa will blow in a few months or years, but "in some sense an eruption is long overdue," says Segall. Luckily, nature will sound a warning. During magma's final push to Earth's surface, it sparks earthquakes. "When eruption is imminent"--meaning two to 12 hours away--"we expect to see an increase in small earthquakes." Even better news: No one has died in any of the 33 Mauna Loa eruptions since 1843.