Harvard University researchers have created metallic hydrogen, nearly a century after it was first theorized. In addition to helping scientists answer fundamental questions about the nature of matter, the material could have a wide range of applications, from room-temperature superconductors to powerful rocket propellant. A room temperature superconductor could change the transportation system, enabling magnetic levitation of high-speed trains. In their experiments, the researchers squeezed a tiny hydrogen sample at 495 gigapascal (GPa), or more than 71.7 million pounds per square inch - greater than the pressure at the center of the Earth. At such extreme pressures, solid molecular hydrogen, which consists of molecules on the lattice sites of the solid, breaks down, and the tightly bound molecules dissociate to transform into atomic hydrogen, a metal.