Undercity, Capitol

On Gallifrey, an undercity was composed of a series of ancient tunnels beneath the Capitol. (COMIC: Supremacy of the Cybermen)

Behind the scenes
In The Runaway HiFi, a short piece of fiction released on Paul Magrs' blog, its revealed that Panda met Iris Wildthyme in a pub in the undercity of Gallifrey. Due to the unlicensed use of Gallifrey, however, it's out of the scope of this Wiki.