This is a story of a gas acting in a way it shouldn’t.

For the first time, physicists from Harvard University turned hydrogen into metal. It proves a concept that was first predicted over 80 years ago by physicists who won the Nobel prize for their work.

Hydrogen, the tiniest element on the periodic table with just one proton and one electron per atom, is a gas at room temperature and normal atmospheric pressure. Usually, hydrogen atom will find another and form a stable molecule. These molecules like to quickly bounce around and take up a lot of space—which is what makes them a gas. (It’s actually rare to find hydrogen on its own in our atmosphere; because it weighs so little Earth’s gravity doesn’t hold it down well.)

But it’s possible to change the state of any element by speeding up or slowing down their molecules, or by giving them more or less room to bounce around. The lower the temperature, the slower molecules move. In this case, the researchers slowed these molecules way down to the point where they were barely moving by cooling them to 5.5 Kelvins (-267.65 °C). At that point, the hydrogen molecules arrange themselves in a lattice structure. Then, they squashed the molecules together to 495 gigapascals, which is nearly 5 million times the pressure we feel at sea level.

When super-cold hydrogen molecules get really close together, something strange happens. “Eventually [the molecules] get so close that two atoms that are in a molecule can’t distinguish whether they should be in that molecule or the adjacent one,” says Isaac Silvera, the lead author of the paper published (paywall) Jan. 26 in the journal Science. Instead of having separate molecules in the lattice, the atoms form a tightly packed mass that all share their electrons—just like a metal.

The research team tested the hydrogen’s metallic properties by shining a light on the sample and measuring how reflective it was. Although they can’t tell if the cold, condensed hydrogen was a liquid or solid, low-energy laser beams bounced back like the scientists would expect them to if the surface was a metal.

To make the metallic hydrogen, Silvera and his colleague, Ranga Dias, used an apparatus called a diamond anvil cell, which, as its name suggests, uses diamonds to apply pressure to chemical samples. “Diamonds are the hardest material we know,” says Silvera. They can withstand a lot of pressure, and are transparent to light and X-rays, which makes the smooshed sample inside the anvil cell easier to study. In this case, they used synthetic diamonds, which have no impurities. They were also coated with a thin layer of aluminum oxide to prevent the hydrogen molecules from seeping into the gemstones.

Silvera thinks that because of the way molecules rearrange themselves in metallic hydrogen, there’s a possibility that when warmed up again it could maintain its structure and be a superconductor. Metals like copper found in wires can conduct electricity, but some of the energy is lost to heat (think about how a lightbulb with filament is hot to the touch). Superconductors can transport electricity without losing any energy, but have only existed at very low temperatures so far, making them impractical for most uses, since keeping things that cold requires lots of energy. In the near future, Silvera plans to conduct research to figure out if, in fact, metallic hydrogen is a room-temperature superconductor.