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.

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 base goes as deep as 1500 meters in some parts. 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 plumbing and toilets.

The Monster Association headquarters are, to some extent, derived from tunnels which were already made by the Subterranean People and the giant centipedes. Statues and other architecture made by the Subterraneans can still be found. The Monster Association massacred the Subterraneans and took the place for themselves, submerging their countless corpses in a large lake deep within 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, and retreat with him to their hideout. Gyoro Gyoro and Orochi remain within the hideout while their army of monsters wreak havoc.

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

Monster Association Arc
Garou discovers a tunnel to the Monster Association headquarters and uses it to enter the base, killing monsters in his way to rescue Tareo.

The Hero Association forms a team and attacks the Monster Association headquarters.

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 presence of corpses and statues of the Subterranean People indicates the Monster Association base was previously inhabited by them.