Undercity

An undercity was the subterranean counterpart to a settlement. Megropolis One, Pluto's first city, had an undercity that became home to outlaws who shunned the society of the Company. (TV: The Sun Makers)
New New York had an undercity that contained many drug vendors. (TV: Gridlock)
On Gallifrey, an undercity was composed of a series of ancient tunnels beneath the Capitol. (COMIC: Supremacy of the Cybermen)
The similar term "Undertown" was used to denote the populated areas of Earth's surface in the 30th century; each Undertown was located beneath a hovering Overcity. (PROSE: Original Sin)