Further testing caused the diamonds to break and the vice to fail, and the researchers haven't been able to find a trace of the metallic hydrogen since.
That doesn't necessarily mean it's been destroyed - the sample was only around 1.5 micrometres thick, and 10 micrometres in diameter - a fifth the diameter of a strand of human hair - so it's possible it's stable somewhere and missing.
But it's also a possibility that, once the pressure of the diamond vice broke, the hydrogen dissipated back into a gas, which suggests that the material isn't stable at room pressure.
The team was preparing to pack up the sample and move it to the Argonne National Laboratory in Chicago for further testing.
Silvera and his Harvard team claimed to create solid metallic hydrogen October last year, using two synthetic diamonds as a type of vice to squeeze the sample.
As the pressure increased, the researchers actually saw with their own eyes as the sample turned from transparent, to dark, and then to shiny and metallic.
It was a huge deal, not just as a proof-of-concept, but because metallic hydrogen is predicted to have some pretty crazy and useful properties - like being a superconductor, capable of carrying current without resistance.
The team kept the sample in the diamond vice at extremely cool temperatures, and conducted initial tests in the lab. Importantly, they measured the reflectivity of the sample to confirm that it was metallic.
They also shone a low-powered red laser into the set-up to measure the pressure, calculating that it was between 465 and 495 GPa - around 4 million times more pressure than the atmospheric pressure at sea level on Earth, and nearly 20 times the pressure initially predicted would be required to achieve metallic hydrogen.
But there were a lot of tests they didn't do. Wary of destroying their sample before the journal article came out, the team didn't measure if their metallic hydrogen was a liquid or a solid.
They also didn't measure whether it could conduct electricity, which is an important feature of metals.
As a result, there's been a lot of skepticism and controversy over whether they'd even made metallic hydrogen in the first place.
To conduct further tests, Silvera and his team were planning to ship the sample to the synchrotron at the Argonne National Laboratory. Before they sent it off, they used the low-powered red laser to measure the pressure of the system once more.
But this time, the energy from the laser immediately destroyed the system, and caused one of the diamonds to disintegrate.
"As soon as we turned the light on, 'click', the diamonds broke. One of them catastrophically, it just became powder," explained Silvera.
"It's one of the things we knew had happened to other teams, but we thought we'd been safe. We'd already tested it before, but evidently something changed over time. Perhaps defects developed in the diamond, perhaps there was diffusion of hydrogen. We don't know what happened."
Silvera is confident that they'll now be able to make more metallic hydrogen - if not in this next round of experiments, soon afterwards.
And he hopes that repeating the process will help to convince some of the doubters.