Subterranean People

The Subterranean People (地底人, Chitējin) are the self-proclaimed "True People of the Earth". They are ruled by the Subterranean King and live under the earth's crust, but grew too numerous in numbers and planned to take over the surface. They were stopped and chased back into the ground by Saitama.

Overview
The Subterranean People have a relatively humanoid appearance. They only have two toes on each foot, and three fingers on their hands. They wear golden or metallic ornaments, wristbands, necklaces, and armor over their right shoulder. They all wear different metal helmets, with only their round eyes showing. They can range greatly in size, from short, tubby little ones to towering, muscular giants.

The Subterranean People seemed rather confident in their ability to take the Earth for themselves, but after the Subterranean King was defeated they retreated quickly, showing their cowardice. They are crafty builders, having constructed the entire base where the Monster Association now resides before their slaughter.

In Saitama's dream sequence they were much larger and more powerful than they truly are, capable of giving Saitama the fight of his life with their immense strength and numbers. Their heights ranged from twice that of an average human to skyscraper-sized. Despite this only being only a dream and their size and strength being exaggerated by Saitama's imagination, there truly exist larger variants of Subterraneans than the ones Saitama encountered in reality, as seen when Phoenix Man resurrected many of their corpses during his battle with Child Emperor.

Saitama Introduction Arc
Saitama wakes up in his apartment when a hand comes through his wall and grabs him, throwing him into the street. Saitama then meets the Subterranean People, who brutally attack him and challenge his strength. They claim that they've already killed many of the humans and proclaim ownership of the Earth. A massive fight ensues, with Saitama eventually being the victor. At that point, the Subterranean King shows up, but before anything could come out of that, Saitama wakes up, realizing that it was only a dream.

Shortly after Saitama wakes up, the real Subterranean King shows up, exciting Saitama. He rushes outside to fight, but the Subterranean King goes down after one kick. Subterranean People are revealed to be much smaller and much, much weaker than in Saitama's dream, and they quickly retreat back underground.

Monster Association Arc
During Child Emperor's fight with Phoenix Man, the two end up in a large pit full of corpses of the Subterranean People. Phoenix Man explains that they were not unified in their ideals, and ended up in-fighting and killing each other.

The corpses of the Subterranean People are reanimated by Phoenix Man to fight for him, but are eventually all defeated by Child Emperor and Saitama.

Powers and Abilities
In Saitama's dream, the Subterranean People were responsible for the greatest fight of his life, possessing the strength, endurance, and numbers to keep up with Saitama's own incredible power. However, in reality, they are not nearly this powerful. Despite this, they do still have disaster levels ranging from Tiger to Demon, showing that they truly can be quite dangerous. They are stronger than regular human beings, and their most dangerous body parts are their feet because they adapted to moving through the hostile conditions of the underground.

Immense Strength and Durability: In Saitama's dream, they were strong enough to make Saitama bleed with a single punch. They were also so tough that Saitama required serious punches and kicks to damage them. They stated no human before Saitama could take one single punch from them and live. However, in the real world, they are neither as strong nor as big as they are in Saitama's dream. Instead, they are all quite small, so much that they didn't even try to fight Saitama and quickly surrendered.

Immense Reflexes, Senses and Endurance: In Saitama's dream, each one of them was able to stand up to the bald hero and keep a fight for a long time. Saitama had a hard time bringing down each one of them and almost died because of his injuries during the fight. They aren't nearly as capable in the real world.

Fighting Style
Hand-to-Hand Combatant: Each Subterranean in Saitama's dream is well versed in hand to hand combat. They are skilled enough to fight Saitama on equal terms and give him trouble, they are also incredibly coordinated and make use of their brute force to dominate Saitama on combat. It is not clear how skilled they are in reality. Swordsmanship: Some of the Subterranean People were equipped with swords, which could be seen all over their corpses in the graveyard.
 * Subterranean Kick (地底蹴り, Chitei keri): A Subterranean kicks with great force. Sometimes multiple Subterraneans will kick together to combine the power of their kicks.
 * Cooperative Attack (連携攻撃, Renkei kōgeki): Multiple Subterraneans surround their target and attack at the same time.

Trivia

 * The Subterranean People originally lived in what is now known as the Monster Association Headquarters under the Z-City Ghost Town, but after their leader was killed by Saitama in their attempt to invade the surface, their unity became fragile and eventually broke resulting in the monsters' self-extinction via infighting. Sometime after this event, the Monster Association took the lair for themselves.
 * In the original version of Chapter 100, some of the Subterranean zombies were much larger in size and wielded gigantic blades against Brave Giant.
 * Certain aspects of the Subterranean People might have been influenced by Mesoamerican civilizations:
 * Their highly detailed masks are vaguely reminiscent of Aztec ceremonial masks or recall depictions of Aztec deities.
 * They ended up killing each other after the death of their leader, and their territory was eventually invaded by another faction.
 * They intended to move somewhere else due to soil exhaustion and overpopulation, like the Mayan civilization.

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