Catacombs

The catacombs constitute a large underground collection of buildings, chambers, and passageways. How far the catacombs stretch and how deep they go is unknown, but at least a portion of it is located in the USA, specifically under Monster High, under the nearby residence areas which include the human town New Salem, and there's a tunnel that reaches the ocean. The catacombs are terrae nullius, as the only known creatures occupying it are not of the sapient kind. The two people who are most experienced with the the catacombs are Operetta and Mr. Hackington, who spend large portions of their free time there.

History
No one knows who built the catacombs, or when, or why so much knowledge of them has been lost. The catacombs were already a mystery a few centuries ago, when Frankie Stein's father set up a laboratory around the time that thirteen students went missing. And about a century ago, Robecca Steam's father joined an expedition into the catacombs that ended up missing too.

The catacombs are largely dangerous, but they are safe to enter as long as people stay within the known portions of the catacombs. Since the area belongs to no one, Monster High has claimed portions nearby the school for facilities the campus doesn't have the room for.

Layout
Since most of the catacombs are forgotten terrain, the layout is only partially known and what percentage that part represents is a mystery. There are both walkways and waterways, including leftbehind boats, to journey through the catacombs. Areas in the catacombs that are not or no longer a secret include:
 * The Pit of Horror (presumably) : A pit that appears to be located in the catacombs, and if so close to Monster High. Some sort of tentacled creature lurks there and the pit occasionally needs to be cleaned. This task can go to students who volunteer, despite the risk of encountering the creature.
 * The Cauldron of Eternal Body Odor : A small lake containing a concoction that smells terrible and which will stick to anyone unfortunate enoguh to fall in for a long time. The lake is located in a large room.
 * The Hall of Halloween, including its own mechanical host : The Hall of Halloween is a construction dedicated to preserving the origins, story, and spirit of Halloween as a holiday celebrating monster-human relations. Most of its features run on electricity, most importantly the host - a skeleton moved by a mechanical arm.
 * A radio station : C.A. Cupid has her radio station located in the catacombs. From there, she hosts a show on love advice for the students of Monster High.
 * A laboratory : When Frankie Stein's father attended Monster High, he created himself a laboratory in the catacombs. This laboratory was locked down when he graduated. Recently, Frankie and her friends reopened it and retrieved Mr. Stein's childhood pet there.
 * An opera house : While the opera house within the catacombs is in use, it is most often deserted bar for Operetta and Memphis "Daddy O" Longlegs. Operetta uses the opera house to play music without anyone bothering her.
 * A soundproof recording studio : Due to the special qualities of her voice, Operetta looked for and found herself a soundproof recording studio in the catacombs. It is illuminated by a special kind of moss growing there.
 * A Skultimate Roller Maze course : A recent creation, the Skultimate Roller Maze course in the catacombs is a favored destination of students looking for either SKRM training or fun by their own rules.
 * A train station : A train station located within the catacombs allows even students living far away to reach Monster High easily.
 * A zombie path : The zombies have a path within the catacombs reserved for themselves for travel between Monster High and their part of town.
 * A deserted castle : An old castle on an island in the middle of a lake is located not too deeply in the catacombs.