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For Kanani Lee, a geophysicist working at Yale, the key to reconstructing the earth’s history is high pressure. Her lab is known as the X-cubed lab, which stands for “Exploring Extremes Experimentally.” The lab studies a little-known layer near the planet’s core, about 1,800 miles beneath the surface and 257 times deeper than the deepest place on the ocean floor. At that depth, the temperature is about forty times hotter than a Finnish sauna. The pressure there is about twenty times that required to make coal into a diamond.

If you were to dig a tunnel to the center of the Earth, you’d first go through a layer of rocks and dirt five times as deep as Mount Everest is high. Eventually you’d hit the mantle, a wide layer of magma about as solid as warm wax, the molten rock rising and falling slowly as it gains and loses heat. Some radioactive elements would probably ooze by you. Then, before reaching the layer of liquid iron that surrounds the Earth’s solid core, you’d pass through something else.

“We think we know what’s down there, but it’s with your eyes closed and a blindfold on,” Lee said. Geologists just can’t figure out what goes on between the rocky mantle and the nickel-and-iron core—whether that layer of Earth is liquid or solid, whether it’s cooling down or heating up, and what exactly it’s made of.

“There’s a big debate right now about how hot it is down there, with a discrepancy of about 2,000 degrees Kelvin. That’s a drastic uncertainty,” said Jung-Fu Lin, a mineral physicist at the University of Texas at Austin.

The layer between the mantle and core might be the source of heat that drives magma to the surface, causing volcanic eruptions. Ultimately, if researchers can understand this layer, then they might be able to reconstruct how deep and hot the ocean of molten rock that covered the earth’s surface during its infancy was once. “We want to know what the earth was like as a baby,” said Zhixue Du, a graduate student in Lee’s lab.

Lately, Lee and Du have been squeezing and heating various metals and compounds that geophysicists know exist at the core-mantle boundary. “The melting point is above 3,000 degrees Kelvin, so just getting these temperatures is hard,” said Lee. That’s more than hot enough to melt thermometers. As Du says, “You cook it.”

Right now, Du is researching magnesium oxide, one of the most abundant materials in the Earth’s mantle, and likely one of the compounds that was floating around in the scalding primordial mess billions of years ago. He wants to know just how hot and pressurized the material can get before it melts. “You can simulate when the earth was molten and see how the atoms are moving. That’s what we’re hoping to get,” said Du.

Du takes microscopic specks of the compound, which look like particles of yellow Jell-O! under the microscope, and wedges them between two diamonds set in steel cylinders. Then he tightens the bolts on the cylinders until the slab of oxide is under extremely high pressure—about 1.4 million times the ambient pressure in which we live and breathe. He fires lasers at the oxide to heat it. The experiment only lasts about a tenth of a second.

The planet’s interior is currently the subject of heated scientific debate all over the world. One group has set up seismometers at the South Pole to measure earthquake vibrations that might reveal clues about the Earth’s inner structure. Many, like those in Lee’s group, are trying to find out how certain materials behave at extraordinary temperatures and pressures. Some of the other methods used to create these conditions involve contraptions the size of a car and ten-meter long guns that create high-pressure shock waves with a tiny particle bullet.

“If you want to understand conditions billions of years ago, you can actually go to the lab and simulate them,” said Du. “We’re getting toward understanding the evolution of the world.”

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