Scientists achieved the "holy grail of high-pressure physics" last month, when physicists from Harvard University claimed they'd successfully turned hydrogen into a metal - something researchers had been struggling to achieve for more than 80 years.

And not only had they made the material, but they were also the first to stably keep it in the lab, making it the only sample of metallic hydrogen anywhere on Earth. But now the team has bad news - the sample has disappeared.

The metallic hydrogen was being stored at temperatures around 80 Kelvin (-193 degrees Celsius and -316 degrees Fahrenheit), and at incredibly high pressures between two diamonds in a type of vice.

But further testing around a week ago 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 - one of the material's predicted properties.

Team leader Isaac F. Silvera, who has spent more than 45 years working on metallic hydrogen, said for now they can't speculate on the fate of the sample.

"Basically, it's disappeared," he told ScienceAlert over the phone. "It's either someplace at room pressure, very small, or it just turned back into a gas. We don't know."

He admits to being disappointed, but they're now focussed on creating an improved diamond vice, and hope to produce another metallic hydrogen sample in the coming weeks.

"We're preparing a new experiment to see if we can reproduce the pressures we achieved the first time, and reproduce our metallic hydrogen," he said.

The failure happened on Saturday, February 11, when the team was preparing to pack up the sample and move it to the Argonne National Laboratory in Chicago for further testing.

So why is metallic hydrogen such a big deal? As the name applies, the material is a metallic form of hydrogen.

Hydrogen is one of the best-studied elements in the Universe - and in its natural state, it's definitely not a metal. It's not shiny and it doesn't conduct electricity.

But back in 1935, researchers predicted that under certain high-pressure conditions, hydrogen could take on metallic properties.

Ever since, scientists have been trying to make metallic hydrogen in the lab - something that's proven difficult due to the ridiculously high pressures they need to maintain.

But Silvera and his team finally managed to do it in 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 material also stores so much energy in its bonds that it could be used as "the most powerful rocket propellant ever discovered".

While waiting for their research to be published in the journal Science last month, 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.

"I don't think the paper is convincing at all," Paul Loubeyre, a physicist at France's Atomic Energy Commission in Bruyères-le-Châtel, who wasn't involved in the research, told Nature last month.

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.

"This disappearance doesn't say anything about the validity of the sample. Anyone who does high pressure works knows that you have failures like this. The important thing is the measurements that we made of the reflectance, and those are solid," he told ScienceAlert.

"So it's not a setback, it's just a disappointment that we were unable to make more measurements on the sample."

"There's always going to be people who are skeptical of things and my advice to them is to try to reproduce the experiment - we've shown exactly what we did to get to the high pressures and achieve the metallic hydrogen in the lab, so other teams can try it too," added Silvera.

"That's the scientific method, and it's better than just complaining about our results."

In the next round of experiments, the team will use a different type of synthetic diamond that will hopefully be more stable.

They've also learnt their lesson, and next time won't keep the sample around for so long before performing further measurements on it.

"It could be if you keep a sample around for a long time it can deteriorate in some way, so once we get a sample up to high pressure next time, we will try to do the important measurements as rapidly as we can," Silvera explained.

We have our fingers crossed for them, and will be watching the results closely.