The Deep End is a chasm in the pool (or pools) of Monster High. It is, as the stories go, bottomless. There are a number of arms within that provide plateaus for various creatures to live on. It is also these arms that stuff that falls into the pool usually lands in, whereas everything that passes by them ends up in places unknown to anyone at Monster High.
The Deep End essentially is the watery equivalent of the catacombs. The two don't appear to be connected but for that the pool is also connected to a lair within the catacombs where a dragon resides that regulates the pool's temperature.
The Deep End is a place the students of Monster High favor to avoid, to the point that merely uttering the name can cause extreme reactions of fright. It is commonly accepted that that which is lost to the Deep End is gone forever, or at least from its previous owner. These attitudes may have been softened a little following the events of "Abyss Adventure", in which Frankie Stein and Lagoona Blue went in to retrieve Draculaura's necklace. Frankie's charge managed to subdue one of the bigger monsters—a squid—inhabiting an arm, which then agreed to help retrieve lots of items the students had lost to the Deep End over time.