Scientists hailed it as the “holy grail” of high pressure physics when they finally produced it in the lab: metallic hydrogen, a century after it was first theorized to exist. And now that sample, which had been held in a hyper refrigerated laboratory, has vanished into thin air, and scientists can’t figure out why.

Hydrogen is the most abundant element in the universe, but it exists almost exclusively as a gas. Theoretically, it could be turned into a metal at extreme pressures, but no one had pulled it off until scientists at Harvard University announced they did it in January, prompting some skeptics and a lot of headlines. But now they say a laboratory accident has caused it to vanish.

Reports indicate that the metallic hydrogen had been kept between a vice of two diamonds at huge pressures while being stored at 80 Kelvin, but something happened in the lab and the diamonds broke. Now, the metallic hydrogen, which was 10 micrometers in diameter, has disappeared. It’s possible that it’s simply right under their noses, but there’s also a possibility that it has turned back into a gas.

“Nearly a century after it was theorized, Harvard scientists have succeeded in creating the rarest – and potentially one of the most valuable – materials on the planet,” Harvard said in their January statement announcing the discovery. “The material – atomic metallic hydrogen – was created by Thomas D. Cabot Professor of the Natural Sciences Isaac Silvera and post-doctoral fellow Ranga Dias. In addition to helping scientists answer fundamental questions about the nature of matter, the material is theorized to have a wide range of applications, including as a room-temperature superconductor. The creation of the rare material is described in a January 26 paper published in Science.”