Since the 1950s, scientists have been trying to drill through the oceanic crust to expose the mantle below. Although that goal remains out of reach--the crust is more than four miles thick--a new drilling project at the bottom of the Pacific Ocean has reached almost a mile below the sea-floor and exposed what lies beneath the uppermost layer of crust for the first time.

A team of American and Japanese scientists began drilling at the site--500 miles west of Costa Rica--in 2002, lured by theoretical predictions of thin crust in areas where the sea-floor spread most rapidly. More than 12 million years ago, this region of the Pacific formed new crust at nearly nine inches per year--faster than any spreading occurring today. Because of that quick spreading, the underlying layer of gabbro--coarse-grained, black volcanic rock--should be nearer to the surface, explains team member Doug Wilson, a geophysicist at the University of California, Santa Barbara. "If that theory were to be correct then we should only need to drill a relatively shallow hole compared to anywhere else," he says.

Ultimately, after five months at sea drilling, that relatively shallow hole reached nearly a mile deep. But at roughly 3,900 feet below the ocean floor the drill bit penetrated the overlying crust rock--baked to extreme hardness by the magma beneath it, like tempered steel--and reached gabbros. This marked the first time gabbros have been recovered intact from the depths of the ocean's basement.

Because such formerly oceanic floor currently covers 60 percent of the earth's surface, the core should elucidate how our planet's crust forms. "Having this sample from the deep fossil magma allows us to compare its composition to the overlying lavas," says team member Jeffrey Alt of the University of Michigan. "It will help explain whether ocean crust, which is about six to seven kilometers thick, is formed from one high-level magma chamber or from a series of stacked magma lenses."

The scientists hope to drill deeper yet at the site, uncovering more information about crust formation, but for now the penetration of this rock boundary represents a major milestone. The researchers report their preliminary findings in a paper published online today by Science.