For the first few million years of our planet’s history, the ocean of molten material that surrounded Earth’s core had two distinct layers, with the deepest one being almost twice as dense as the one nearest the planet’s surface, a new study suggests. Researchers gathered data by squeezing samples of the volcanic rock basalt (dark blob in center of image) between small bits of diamond at pressures approaching 600,000 times that exerted by Earth’s atmosphere at sea level—or about the same as that found within rocks about 1400 kilometers below Earth’s surface. (The tiny diamond anvils are one of the few materials strong enough to exert such pressures without shattering.) While squeezing the samples, the team also heated them to about 3000°C by zapping the materials with pulses of high-energy x-rays—a first-of-its-kind combination. At the relatively low pressures found near Earth’s surface, basaltic magma would have had a density of about 2.7 grams per cubic centimeter, the tests suggest. But at the hellish temperatures and pressures found at a depth of 1400 kilometers, its density probably would have been about 5 grams per cubic centimeter , the researchers report online today in Nature. The increase in density, and a corresponding increase in stiffness of the molten material, results because the high pressure found deep within the planet would have forced each silicon atom to have six oxygen neighbors, not the four neighbors typically found at lower pressures. These magma layers probably lasted no more than 100 million years, but they strongly affected the rate at which our planet cooled, as well as how various elements were distributed within Earth’s crust and the underlying layer known as the mantle, the researchers say. Although scientists have theorized that Earth’s early oceans of magma were two-layered, the new results are the first solid evidence to bolster that notion.