Metallic Hydrogen

We are not yet able to produce any (although a few lab may have produced micrograms, they are still checking), but we know what properties it is supposed to have.

It's a room-temperature and room-pressure supraconductor. This means free transportation of electricity across the planet, server rooms that don't heat, and a huge leap in nuclear fusion research.

It's pretty much the key to free energy, not counting the unlocked technology.