Monster Association Headquarters

The Monster Association Headquarters is the hideout and base of operations of the Monster Association. The base is immense, housing over 500 monsters, and leads out to multiple cities through the tunnels created by Elder Centipede. During her fight with Psykos and Orochi, Tatsumaki lifted the entire base out of the ground.

Overview
The Monster Association's base is subterranean in nature, lying deep beneath the Z-City Ghost Town. The Monster Association's presence is a large factor why the ghost town is abandoned. It is largely vertical in shape, almost like an underground tower of sorts, with countless tunnels and staircases connecting its levels. The main hall of the base is Orochi's throne room, where the Association gathers for briefings and other matters. In its maze-like tunnels are other facilities such as prisons, where hostages are held, and mines, where worker monsters mine resources such as gold and jewels. It also has functioning toilets and elevators.

Hidden even further beneath the headquarters, approximately fifteen-hundred meters under the surface, is Psykos' secret chamber. It doubles as her place of rest and a personal laboratory. She claims that no monsters know of its existence. From there she controls her meat puppet Gyoro Gyoro, who in turn leads the entire Monster Association. In order for her psychic waves to reach the rest of the headquarters, Psykos had to install psychic relay points throughout the base.

The Monster Association headquarters is derived from tunnels which were already made by the Subterranean People and the giant centipedes. After the Subterraneans drove themselves to extinction via infighting after the loss of their leader, the Monster Association took the base for themselves, leaving the many corpses of the indigenous people buried deep in the base.

Monster Raid Arc
The Monster Association attack all cities to create a diversion to kidnap the son of an important Hero Association sponsor as a hostage. The Association then retreats with Elder Centipede covering them, and although a Metal Knight drone follows after them to reach the headquarters, the drone is destroyed by Orochi and strung up in the throne room.

Super Fight Arc
After Gouketsu defeats Bakuzan, he heads out to return to the headquarters but is killed by Saitama before he can reach it.

Monster Association Arc
Garou is captured by Phoenix Man and taken to the Monster Association headquarters, where Gyoro Gyoro tries to recruit him to their side. They then release him back to the surface, with Royal Ripper and Bug God following him. Garou later discovers a tunnel back into the Monster Association headquarters and uses it to enter the base, fighting monsters in his way to kill Gyoro Gyoro and rescue Tareo. His encounter with Overgrown Rover destroys a large hall in the base and sends him careening into Gyoro Gyoro's private chamber. Garou then battles Orochi in the throne room, which opens up using large mechanisms to accommodate Orochi's massive size. After Garou defeat, he is taken deeper underground and chained in the prisons. Saitama uses the sewers to investigate the sounds from Garou and Orochi's battle, eventually reaching the Monster Association headquarters.

The Hero Association forms a team and attacks the Monster Association headquarters. They defeat the monsters who meet them on the surface before splitting up to attack from different angles underground. In the battles that ensue, large portions of the headquarters are destroyed because of explosive fighters such as Child Emperor's Brave Giant, Orochi's final form, and Overgrown Rover.

After confirming Tareo is safe with King, Tatsumaki lifts the entire base to the surface.

Trivia

 * In Chapter 115, Saitama walks through a section of the Monster Association base that bears a close resemblance to a section of the Taman Sari Water Castle in Indonesia.
 * The Monster Association Headquarters being originally inhabited by the Subterranean People, and subsequently stolen from them after being massacred by Orochi, was an idea given to Murata by Lee in one of his livestreams.