The Diggers were humans gifted by powers by the Epic Digzone.
Though the gifted powers of the Diggers was not clear, they did have the ability to dig through solid steel.
Digzone gifted a group of humans, employed by Steelheart, with powers allowing them to dig through steel. Steelheart made them dig the Understreets of Newcago. Eventually, the Diggers went insane and created the Steel Catacombs, a labyrinth of tunnels, bad ventilation and electricity. This proves that not all Epic powers can be gifted to humans.