In Victorian time Michael Faraday gingerly liquefied gase inside thick-walled glass tube. By World War I, ingenuity and boldness of design allowed Harvard' Percy W. Bridgman to reach much higher pressure along semi-industry lines. His large press disclosed innumerable new phase in familiar condensed matter, most famously several new crystalline phase of H2O. Eventually he came to water ice VI, which at the highest pressure of the time remained an ice solid even at the temperature of a very hot kitchen oven. The price of this mastery was not low: in 1922 a pressure container failed, steel flew like shrapnel and two men were killed, although the Harvard laboratory went on safely for decade more. Step by step, industry developed high-pressure technology for large-scale synthes. The most important early product was ammonia, made in the million of ton by reacting hydrogen and nitrogen gase at modest pressure. The most celebrated is synthes diamond, by now a high-pressure commodity. Set some simple marker along the pressure scale. Infant breath and puff their cheeks, displaying small change in local air pressure. Mountaintop lure other among us into pressure lower by a factor of two or so. The weather bring stay-at-homes incessant , less noticeable variation in pressure, but sea-level air still afford us a meaningful natural unit. Call the typical pressure at sea level one atmosphere. Like many another kid, I was disappointed year ago by the poor result of taking a long rubber tube into the pool, in an attempt to tap surface air for breath on the bottom. No way! The added weight of even a foot of water is too much for chest muscles. To breath at any depth underwater, you need either to be housed within some sealed submersible or to sip breath of air fed from a portable tank at regulated matching pressure. That is SCUBA-self-contained underwater breath apparatus. The ocean define a second natural pressure marker. Call 1,000 atmosphere one "ocean," by analogy. That value is a ballpark fit to seawater pressure six mile down in the deepest undersea trenches. The earliest physical successe called for pressures reaching one ocean or so, and the ice for pressure half a dozen time greater. The diamond appeared after World War II up at 50 or 100 oceans, at which graphite dissolved in molten metal at white heat crystallize to diamond. Such condition have become routine in the special multiton presse that yield synthetic diamond around the world. Solid rock is denser than water, and our planet' center lie a decisive 1,000 times deeper than the average-depth sea bottom. We can plausibly define a third natural benchmark for core pressures, one equal to 1,000 ocean of 1,000 atmosphere each, or a million atmospheres. Calling one atmosphere by the crisper name "bar," it informal equivalent, we use it multiple, one megabar, as a convenient unit for describing the tremendou pressure at the core. It turn out to be surprisingly easy to estimate the pressure so deep down. The symmetrical Newtonian pull of all the interior part on all the layer above compact Earth into a near sphere. Even the expected increase in density as depth approache the center has at most a modest effect on a calculation that treat the globe as a uniform fluid. The central pressure approximate the simple product of three well-known quantities: the average density of Earth; the planet' radius; and the familiar constant g, the value of surface gravity signaled by every apple that falls. That central pressure is reliably estimated at about four megabar whatever the internal composition, if only all the strata are both stable (not undermined by vast buried caverns) and uniform from place to place, differing only as their depth differs. What is down there anyhow? This rich question bear strongly on the literal root of all geology. Can we proceed with theory alone as guide? Can we ignore unknown minerals and fluids, some sure to be as strange as hot ice was? The geophysicist wisely chose to experiment. One megabar of pressure was first reached 20 year ago at the Geophysical Laboratory of the Carnegie Institution of Washington. That lab and other can now summon the conditions of Hade itself right to the bench top. The megabar apparatu in no way resemble the heavy, house-size hydraulic presse of old, stowed behind safety wall and served by a team of cautiou engineers. It resemble more a top-of-the-line research microscope in the precision machining of it minute, elegant part and is not unlike it as well in bulk, weight and cost. No monster machine at all, this device is mostly made in house by a few master machinist as the prized tool of a few physicists. How do they evade danger, weight and expense? Pressure is a local quantity. Even a handheld steel sewing needpushed firmly into a surface can exert oceanic pressure if only over a tiny area. Megabar of pressure in a minute container, a tenth of a millimeter on edge, store up something like the energy released by a pencil falling off the workbench. If that holder fractures, expect only a small pop, a source more of sharp fiscal pain than of physical injury. Work small; a miniature-scale apparatu produce pressure to match a planet' core simply by hand-tightening a single bolt! But can you learn about Earth' center from so small a sample? Indeed you can, for matter is atomic. Its propertie are well developed whenever sufficient atom are present long enough to arrange themselve as the laws require. That small pressure vessel holds some million billion atoms, more than enough to allow matter to fulfill its subtlest intentions. A few microgram are ineffective for making diamond commercially but brilliantly suited to yield knowledge. How will you learn what happen to matter compressed under megabar? First, use a transparent container, just as Faraday did. Probe it adroitly with infrared and optical lasers, with x-rays, neutrons, microwaves, even sound. Tight, high-intensity beam may be needed for a scant sample; for x-ray diffraction studies, the geophysicist often subject their pressure cell to a fierce synchrotron source. The limit are set mainly by the changing state of condensed-matter physics. How to contain the sample? Squeeze it inside a tiny box of the hardest stuff known, diamond. Find two flawless gems-single crystal selected for purity and extratransparency-then cut in facet to order. Less than one carat in weight, they will be dear but affordable. Align the tip end of the two truncated diamond cone with a thin ring gasket of steel alloy held between the two tiny flattened tips. The sample is confined within the hole in the gasket. Press diamond hard against diamond, and the whole assembly seal tight. Megabar come easily. A diamond-diamond total force of only a couple of hundred kilograms is adequate, for that force is concentrated over an extremely small area. Up-to-date diamond-anvil cell may have contact faces only a thirtieth of a millimeter across. Sophisticated mounting design and virtuoso fabrication keep the gem nearly parallel for final optical alignment. Cryogenic cooling and resistance heating are both quite manageable. The theorist opine that at about 12 megabar, diamond undergoe a change in phase, and some anvil will break. That limit is still ahead. About half that pressure is expected to be enough to press molecular hydrogen into hydrogen metal, a goal that has long eluded the labs. In the past few months there has been a credible report of making the metal hydrogen using a big, noisy gas gun, but the stuff lasted the mere microsecond it took a shock wave to cross the cryogenic hydrogen target. Fortune may favor us with some new anvil substance harder than pure diamond, so we will be able to see the luster of that simplest of metals. In this subtle corner of condensed-matter physics, the pressure of a planet's core is literally at hand, through an art once awkward but now as harmonious as a quiet song.