The Biggest Dig

Japan builds a ship to drill to the earth's mantle

If you've ever thought about digging a hole to China, a new Japanese ship might be your best bet. Workers have just put the finishing touches on an ocean drilling vessel that is designed to bore to unprecedented depths and attain a long-held goal: penetrating the earth's rocky crust to the mantle.

The poorly understood mantle accounts for about two thirds of the planet's mass and is key in the unseen convection processes linked with tectonic plate motion.

For Japan, an archipelago straddling the fractious intersection of at least three crustal plates, the issue is also earthquakes. "Japan is situated on these active planetary processes, and 30 million people actually live on one of the most dangerous or active places on the earth," says Asahiko Taira, director general of the Center for Deep Earth Exploration (CDEX) of the Japan Agency for Marine-Earth Scienceand Technology, which operates the ship.

To study the mantle, geophysicists have had to rely on indirect methods, such as looking at seismic signals and measuring gravitational field variations. They can examine mantle rocks that have been brought to the surface via volcanism or faulting, but because this material has undergone massive amounts of heating, cooling and other processes, many argue it is not truly representative of the mantle. Breaking through the border between the crust and hotter mantle--known as the Mohorovicic discontinuity, or Moho--would give scientists a direct, fresh sample of mantle as well as the fluid, gas, temperature and pressure conditions of its environment (including possible microorganisms) that are lost by the time the rock arrives at the surface naturally. Researchers from 18 countries working on the U.S. drill ship JOIDES Resolution recently tried to reach the mantle at the Mid-Atlantic Ridge, but they missed by less than an estimated 300 meters.