Undercity
From Tardis Wiki, the free Doctor Who reference
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)
The term "undercity" was also used to denote the populated areas of Earth's surface in the 30th century, as opposed to the Overcities. (PROSE: Original Sin)
New New York had an undercity that contained many drug vendors. (TV: Gridlock)
