writes The world's largest laser [the National Ignition Facility], a machine that appeared as the warp core in 'Star Trek into Darkness', has attained a powerful result: It's squeezed diamond, the least compressible substance known, 50 million times harder than Earth's atmosphere presses down on us. ... As the researchers report online today in Nature, the x-ray assault nearly quadrupled the diamond's density. "That's a record," Smith [one of the researchers] says. "No one's compressed diamond to that extent before." The blast pulverized the diamond into dust, but before the mineral's destruction the scientists successfully measured its density ... For a billionth of a second, the diamond, which is normally 3.25 times denser than water, became ... 12.03 times denser than water. ... Scientists have speculated that diamond worlds may exist elsewhere. If a solar system arises with more carbon than oxygen, then carbon should soak up the oxygen by forming carbon monoxide, leaving excess carbon to create carbon planets—which, under pressure, become diamond worlds. Thus, Smith says, the new experiment will probe the nature of such planets.
They are performing similar experiments with iron in an attempt to understand the properties of super-Earth cores.