In Britain, an underground room is a cellar.
An underground room is a room that is located below ground level, typically beneath a house or building. These rooms can serve various purposes such as storage, wine cellar, storm shelter, or even as a hidden room.
The name given to the room beneath a house is called a basement.
underground , it is part of the underground railroad
Cellar Basement
its a subterranean chamber ; a subterranean chamber is a underground room used to confuse robbers
Cellar
A subterranean chamber is a room or open space that is underground.
Carlsbad Caverns.
It was in his ship which is now gone, you had to get the key which was in his office.
All of the secret rooms are... EPF Command Room, Secret Ninja Hideout, Iceberg, Underground Pool, Boiler Room, PSA Head Quarters from older years, Underground Mine, The Hidden Lake, and the Command Room.
Carlsbad caverns
underground pool