The Mole Men are a race of intelligent, subterranean humanoids. They are short, nearly bald, and have mole-like bodies. They seem to posess advanced technology, as is evidenced by the laser weapon they use. They also seem to be radioactive.
The Silsby Riot
The Mole Men were first able to access the surface of the Earth via a deep oil well dug in 1951 in a small town known as Silsby. Silsby was famous for its oil well, which was the deepest in the world at the time. The well went six miles deep, and broke into the Mole Men's habitat; this allowed some Mole Men to escape to the surface.
The sight of these strange creatures, who didn't seem to mean any harm to the Humans, frightened the townsfolk of Silsby, leading to the creation of a mob. The mob angrily persued the two Mole Men. Luckily, reporters Clark Kent and Lois Lane were investigating the town, and Clark-- as Superman -- was able to save the lives of the two Mole Men, although one of them was shot and wounded. Superman brought him to a hospital, where he was treated.
Later, a rescue party of Mole Men came to the surface to retrieve their wounded companion. Superman protected them from Humans, while protecting humans from the Mole Men's advanced weapon; he helped them to retrieve their friend and return to their underground dwelling. They then destroyed the oil well, prompting Lois Lane to comment "It's almost like they were saying, 'you'll live in your world, and we'll live in ours'."