The lowest floor of a building, below the basement.
Subbasements are usually dark and used for storage or mechanical equipment.
Examples:
The subbasement is where we keep all the old stuff.
The subbasement is flooded after the heavy rain.
I don't like going down to the subbasement.
The subbasement is so dark, it's like a cave.
The subbasement goes down and down, like a never-ending hole.
In the subbasement, there are strange noises at night.