"Washing Machine Tub Reservoir" is part of a descriptive lexicon to describe a feature found by Urban Explorers in the absence of proper civil engineering terms.
Whereas most underground reservoirs are accessible through a side-tunnel, the "washing-machine" shape denotes a reservoir that has a central spindle like the agitator in a washing machine.
The structural beauty of this type of reservoir lies in its non-appearance above ground, merely being a grille or a vertical pipe on the surface, belying what lies beneath. The explorer expects only to descend into a shaft or a sinkhole, and upon opening the doors of the spindle, is faced with a magnificent big, empty space that has been hidden from all eyes but the explorer's own.