A dimension containing an infinite Aquarium. There an infinite amount of every type of fish, along with an infinite amount of bathrooms and an infinite amount of merchandise stores with no employees in them.
The dimension's structure is much like that of a normal aquarium, with tanks full of fish and stairs, however there is no exits and instead of employees the fish are taken care of by non-sentient machines. No alarms go off with something normally illegal like breaking a tank is done.
The fish seem to live happy and healthy lives, as the machines never seem to break or run out of food. Any non-fish entities, on the other end, tend to go insane and make bases- breaking the tanks for food and water, and often entertaining themselves by acting as if they are back home and nothing weird happend to them.
Many non-fish entities end up in The Aquarium after getting lost while in water. Some were in a sewer system when there was a flash flood, some were at the beach surfing and got knocked out by a wave, some even just slipped and fell into a deep enough puddle. The entities then find themselves inside of the tanks and have to escape before they drown.
The only known way to exit The Aquarium is by getting lost while in water and then getting lucky. The entities will then find themselves back in the body of water that brought them there in the first place.