a basement..? pretty simple.
If its an underground reservoir for water, it could possibly be an Aquifer.
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.
An underground water storage is called a cistern. It is a structure used to collect and store rainwater or groundwater for later use. Cisterns can range in size from small containers to large underground tanks.
The entrance to underground mines is called a portal or adit. It is the point of access for miners and equipment to enter and exit the mine.
aquifers
They are called pithouses and some rooms are underground.
A burrow. In a burrow there are underground rooms called warrens.
Kiva
Syme was and enthusiatic supporter of the party's decisions about stuff in the underground rooms below the ministry of love Page 13 in the middle
concentric castles could have 3 rooms on each floor it also has a underground where the duguon is kept
'rooms'.
Rooms on a ship are called cabins and suites; floors are called decks.
set of rooms
A group of connected rooms is called a suite.
set of rooms
A rabbit's habitat is called a burrow. They dig underground tunnels and rooms to create a safe and secure living space for themselves and their young. Burrows provide protection from predators and extreme weather conditions.
A group of rooms is called a suite.