Back in 1935, scientists predicted that hydrogen could be transformed into a metal under immense amounts of pressure, similar to the way carbon atoms can form into diamonds. Back then it was thought that 25 gigapascals (about 250,000 times normal atmospheric pressure on Earth) should do the trick.

Harvard physicists Ranga Dias and Isaac F. Silvera say they had to find a way to subject hydrogen to nearly twenty times that much pressure, which is more intense than the pressure at the center of the Earth, before it finally underwent the transition that had been predicted over eighty years ago. 

"This is the holy grail of high-pressure physics," Silvera said. "It's the first-ever sample of metallic hydrogen on Earth, so when you're looking at it, you're looking at something that's never existed before."