Sub Earth Man

Sub-Earth Man Mystic Comics #5 (March 1941)
While exploring a long dormant volcano in Mexico that has inexplicably become active again, geologist Bob Roland and his fiancée, Carol, discover a mysterious stone door that leads them into the lost subterranean kingdom of the Sub-Earth Man, God of the Middle Earth.
Sub-Earth Man is a fairly intriguing character. He's gigantic, about fifty feet tall, with flames all over his body. By contrast, his servants, the faintly demonic looking "Droogies" are normal human size and wear asbestos suits. The Sub-Earth Man plans to use volcanoes and earthquakes to destroy humanity so that he can once again rule the upper world as he claims to have done in the distant past when Earth was little more than a burning fireball.
Bob and Carol manage to escape the Sub-Earth Man's realm and foil his attempt to use an underwater volcano to create a gigantic tidal wave that will wipe out the Mexican coast.
Typical of Golden Age comics, the science was pretty dodgy, but the premise was interesting and had some potential. Unfortunately, after this single tale, the Sub-Earth Man never surfaced again.