Seismic imaging can generate maps of the mantle, but geologists are not going to collect direct samples of those parts of the planet anytime soon. So some researchers are exploring the earth's deep interior by re-creating it in the laboratory. Last year, Stanford mineral physicist Wendy Mao simulated conditions in the upper mantle using a technique that could test theories about the formation of the earth's core. She subjected a cell-size mixture of iron-rich alloy and silicate to 60,000 times the atmospheric pressure at sea level. Then she heated the sample to 1800 degrees C and peered inside it with a series of X-rays to produce this three-dimensional image.

Mao observed that the iron melted into discrete spheres, visible here as bright circles in the darker silicate matrix. In future simulations she plans to crank up both temperature and pressure to find out if iron could have melted and trickled through the silicate mantle to give rise to the core some 4 billion years ago. An iron and silicate meteorite is thought to resemble the raw material from which the core formed.

"But in the lab," Mao says, "we can get a real window into the earth's interior."