At extreme pressures, hydrogen - a gas under normal conditions - takes on a new solid form that is remarkably different from other structures known to science while also disproving a long-held theory, according to the latest research from the Carnegie Institution for Science.

Hydrogen is the lightest and most abundant element on Earth. Up until now, only three solid states of hydrogen were known, though since the 1930s it has been theorized that the element would become a metal at extremely high pressures. Metallic hydrogen has been described as the "holy grail of high-pressure physics," often theorized about, but impossible to confirm because generating the center-of-the-Earth-like pressures necessary to get hydrogen into a metallic state were not technically feasible in laboratories.

But by using a facility maintained by the Geophysical Laboratory at the National Synchrotron Light Source at Brookhaven National Laboratory, the Carnegie team was able to test hydrogen under extreme pressures and temperatures. Their research found that the new form of hydrogen was stable from about 2.2 million times normal atmospheric pressure and about 80 degrees Fahrenheit to at least 3.4 million times atmospheric pressure and about -100 degrees Fahrenheit. Measurements of the hydrogen were taken using intense infrared radiation.

Under these extreme conditions, hydrogen molecules take on two forms. One type interacts surprisingly weakly with its neighboring molecules (unusual at such high pressures). The other type of hydrogen molecule bonds with its neighbors, forming planar sheets. The measurements show that solid hydrogen under such extreme conditions is an unusual hybrid solid, on the border of being a semiconductor, like silicon, and a semi-metal, like graphite.

The results, researchers say, disprove the claims that hydrogen forms a dense atomic metal under extreme atmospheric conditions.

"This simple element - with only one electron and one proton - continues to surprise us with its richness and complexity when it is subjected to high pressures," said Russell Hemley, Director of the Geophysical Laboratory, according to a news release. "The results provide an important testing ground for fundamental theory."