subbasement

noun

sub·​base·​ment ˌsəb-ˈbās-mənt 

variants or sub-basement

plural subbasements or sub-basements

: a basement or an underground story located below the main basement of a building

… describing in loving detail the enormous, complex structure from the bottom up, its subbasement … is full of gleaming pipes, turbines and boilers.

—Burton Bernstein

Word History

First Known Use

circa 1776, in the meaning defined above