First, they placed some hydrogen in an alumina-epoxy gasket and placed that within a diamond anvil cell. This allowed them to test the opacity and electrical resistance of their sample via laser and electrodes, respectively. Then, at room temperature, they dialed the pressure up to 220 gigapascals, at which point their sample became opaque and began to show conductive properties. In the next phase of their experiment, they also dialed down the temperature to roughly -400 degrees while upping the pressure to 260 GPa. Here, electrical resistance increased by 20 percent.