As far as can be told, there are several underearth races living in a network of carverns and tunnels that extend around the world.
The most numerous race is loosely referred to as the Subterraneans. Others include the Deviants, the Lava Men, the Underlords, and the Netherworld Atlanteans. The reptilian Cretacians can also be found underearth.
Apparently a genetic offshoot of the human race gene-manipulated by some elder space-faring race, Deviants are known for never breeding true. Each member of the race is somewhat different from every other. Some are simply deformed, others have significant powers. They are apparently engaged in a centuries-long war with the Eternals, other human offshoots created by the same aliens. Reports from the 70's indicate that the war escalated sometime in the 1930s (some feel that Hitler may have been a Deviant) and, by the 70's, Deviants had largely lost the war. It is thought that Deviant survivors took to hiding amongst humanity. Those who have studied the matter feel that the rash of superbeings since the end of the Second World War may be the result of Deviant genes showing up amongst humanity again.
If so, the large number of heroes resulting indicates that the Deviants are not as naturally evil as the Eternal reports would indicate. Some feel that the so-called "mutants" or "X-people" are Deviant genes coming to the fore.
The Lava Men appear to be simply that, people who can live in lava itself. Unlike most of the other races mentioned, they are an entirely separate line of development and are apparently a silicon rather than carbon-based life form. They came to the world's attention in the 1960s when the Avengers kept them from attacking the surface world, and have existed in an uneasy and sometimes broken peace with the surface ever since.
They apparently have trouble in cold climes and are not very fast breeding. However, they are possessed of great powers of lava and fire control and can hold their own in a fight with any earth army of normals. It is rumored that they participated in the Underearth War of several years ago as aggressors.
When Atlantis (the original, not the various undersea peoples) sank so many centuries ago, one of its colonies involuntarily fled the cataclysm by being sunk in a pocket of earth that lowered itself deep into the earth. There they managed to survive, progress and prosper through trade with the Deviants (some say that Deviant technology brought them to their place) until they were rediscovered by Iron Man.
Since then, they have had a wider impact in the Underearth, but suffered greatly when some of the stabilizing engines failed and much of the city was destroyed by shifting faults. The survivors are scattered about the Underways, mostly in the kingdom of the Mole Man, who has married their Queen Kala.
The subterraneans are thought to be a race either bred to be slaves by the Deviants or Deviants whose wild genetic structure stabilized within them to create this very stable genotype. While almost unintelligent, they are very numerous in the Underways--apparently they breed very quickly. They can also be trained to do almost any manual task with ease and dispatch.
Subterraneans are generally hairless, very short, usually below 4 feet tall, and slim with disproportionately large heads. These large craniums do not hold extra brainpower; they mostly hold a very sophisticated optical system that lets these creatures operate in almost total darkness. This is not infravision or some other system of seeing something in the dark. Instead, they can use any glimmer of light to light up their surroundings. If they would otherwise be in total darkness, they do use lighting of some sort--fire, luminous fungus, chunks of hot lava, whatever.
Subterraneans come in several different colors. The ones most commonly seen are the yellowish-skinned ones who serve the Mole Man. They are called Moloids. Tyrannus led a similar group whose skin was more of an off-white, called the Tyrannoids.
The Underlords, as far as can be told, are a mixture of some adventurous Atlanteans who left the underearth city centuries before its demise and Subterraneans (showing that the Subterraneans were bred from human stock). They tend to be human size but share the Subterranean lack of hair and light-sensitive eyes (though they are not as sensitive as the Subterranean eye).
In the 1960s, a leader called the Overlord attempted to invade the surface world with weaponry based on the latest designs from the surface and improved by Underlord science. It is thought now that Overlord himself was probably a Deviant and that this was a desperate Deviant attempt to take over the earth and turn its entire resources against the Eternals.
UNTIL and THUNDER, with assistance from the rest of the world's heroes, turned back the invasion and eventually killed the Overlord. This loss was a great blow to the Deviant cause, leading to their defeat by the Eternals. Of course, at the time of the war with Overlord, neither Deviants nor Eternals were known.
From what accounts are available, a war broke out for control of the underworld after the defeat of the Deviants and the departure of the Eternals. Two surface men who had taken control of large groups of Subterraneans, the Mole Man and Tyrannus, fought it out for control of Netherworld Atlantis, the remnants of Deviant technology, and the loyalty of the Lava Men, who had their own agenda.
Surface heroes became involved in the struggle and the eventual outcome reported indicated Mole Man was in control and married to Queen Kala of Atlantis, while Tyrannus was dead. The Underlords, apparently still smarting from their defeat in the 60s, had stayed out of the conflict.
The Mole Man and Tyrannus were both intruders from the surface world. The Mole Man found refuge under the surface sometime in the 1940s, after many years of persecution for his looks on the surface. Once in the tunnels of the Underearth he apparently discovered the ruins of a technological society (a former Deviant holding) and the Moloids, who served his every whim.
Tyrannus, on the other hand, is apparently a member of a Roman Imperial family from the time of the Empire. Disgraced in interfamily politics, he found his way to the UnderEarth and started building up a power base among the subterraneans. He found a way of prolonging his life, noble looks and keen intellect for centuries. He, too, benefited from Deviant technology. For centuries he was content beneath the Earth, but in the 1960s he attempted to invade the surface, only to be stopped by the original X-Men.
While not an underearth race (they prefer mountaintops) the Eternals were a centerpiece of underearth policy for centuries. The Deviants devoted every waking minute to the destruction of the Eternals, who were few in number but very powerful.
After their final defeat of the main body of Deviants, the Eternals had time to look elsewhere. They managed to make contact with another race similarly altered by the gene-meddling aliens so long ago and decided to use their great mental forces to meld themselves into one unit and venture forth among the stars to find these others. This, at least, is the story told by the few remaining Eternals who could not bear to leave Earth or were needed to keep an eye on what remained of the Deviants--it depends on whose story is being told when.
As Far As Can be Told, about six years ago, Mechanon masqueraded as Tyrannus to unify the Underearth. Why this very anti-organic mastermind wanted to unify a group of organics is unknown. The only main lieutenant captured, the Lava Champion, refuses to this day to believe that Tyrannus is Mechanon. Apparently there is some kind of cult of Tyrannus Reborn involved. The implication is also that Tyrannus' followers are mostly malcontents from their various cultures. The Lava Champion wants to be king of the Lava Men, the Moleman's Son wants to prove himself independent of his father, and the moloids and monsters are the ones personally loyal to the son. The Champion knows of no Underlord or direct Deviant involvement.
"Tyrannus" altered some of the moloids into cyborgs. This matches some behavior of Mechanon in the past, particularly in Dallas. Apparently he has a tolerance for organics who are willing to give up some of their flesh to metal.
Long before there were Subterraneans of any sort, The Cretacians (as they are called by humans) ruled the dinosaur-ridden world of the late Cretacious age. Mentalist intelligent sauroids, they specialized in biotechnology, breeding their own domesticated versions of many familiar dinosaurs. When the cataclysm that destroyed the Cretacious Age came, different groups attempted to avoid their fate in different ways. Many took refuge deep within the earth. Remnants of this species have appeared off and on for centuries. They were seen in Africa at the turn of the century, in California during the 1940s and in New Mexico in the 1950s. They are reported to have appeared in England in the 1970s as well.
They are invariably hostile to humanity, though they have made alliances with some villainous groups. They are known to have had contact with star-traveling races of their era, about 60 million years ago, though it is unknown whether any of those species are still active in space. A flying saucer incursion in Korea during the Korean War may indicate that they are.
It is thought that some of the underground technology might be the result of subterraneans of one form or another finding the remnants of the Cretacians and stealing their technology, though few of them have used biotechnology to the extent the Cretacians did. Perhaps the Mole Man's monsters were created using Cretacian bio-engineering techniques…