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 catacombs are Operetta and Mr. Hackington, who spend large portions of their free time there.
The catacombs essentially are the nonwatery equivalent of the Deep End. The two don't appear to be connected but for that a lair within the catacombs, where a dragon resides that regulates the school's pool's temperature, is connected to the pool that houses the entrance to the Deep End.
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, such as a train station and its own Skultimate Roller Maze course. As well, Mr. Hackington organizes Creature Spotting field trips into unclaimed portions of the catacombs. Of the flora and fauna to be found there are scargoyles and monster thistle. Dragons and bone elementals are known to occupy the loneliest parts of the catacombs, which means they're only encountered if a full expedition into the catacombs is organized.
Though they aren't known to enter them regularly, the citizens of New Salem are familiar with the catacombs, or at least the portion under the town. This area borders on the monsters' terrain, but the two groups rarely cross over into each other's.
Though the catacombs are an area to be treated lightly where disappearances easily happen and still new areas are discovered with no suggestion of the place being fully mapped any time soon,the places is the most popular location for the Monster High student body to hold parties, closely followed by Monster High itself and at a distance by the De Nile residence.
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 left behind 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 enough to fall in for a long time. The lake is located in a concert hall.
- 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 library: There's a library in the catacombs referred to as "the old library" by the Monster High student body. It contains plenty of books on obscure knowledge, including the layout of the catacombs.
- 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.
- The catacombs were first mentioned in the Dawn of the Dance invitations and in Mad Science by Mr. Hackington and may have first been shown in "Fur Will Fly". Despite this early start, it took until the webisode "Hiss-teria" for the catacombs to become involved in the story of Monster High. Not surprisingly, this went hand-in-hand with the introduction of Operetta (a character whose proper introduction also happened long after the first mention). In this regard, the catacombs are a parallel to the cellars of the Palais Garnier in The Phantom of the Opera, though they've grown much more significant to the Monster High universe than just Operetta's hangout.
- The catacombs have a distinct medieval vibe to them, which is odd since they are located in the USA and of such an age that they can't have been built by the early colonists by real-world standards. Whether this adds to the mystery of the catacombs or means that history happened differently in regards to the American continents in the Monster High universe is yet to be revealed.
- ↑ Mad Science by Mr. Hackington
- ↑ 2.0 2.1 "Fright On!"
- ↑ "Uncommon Cold"
- ↑ C.A. Cupid's SSS diary
- ↑ 5.0 5.1 5.2 "Ghouls Rule"
- ↑ "Fur Will Fly"
- ↑ "Why Do Ghouls Fall in Love?"
- ↑ C.A. Cupid's back-of-the-box bio
- ↑ "HooDude VooDoo"
- ↑ "Phantom of the Opry"
- ↑ 11.0 11.1 11.2 11.3 Operetta's 'Campus Stroll' diary
- ↑ Rochelle Goyle's 'Between Classes' diary