"The Undercity" is how Stormgate's surface-dwelling citizens refer to any of the city's subterranean areas. It's an umbrella term that includes many levels of underground tunnels, ruins, caverns, dungeons, and catacombs.
In part due to the regular torrents of rain water that crash into Stormgate, structures tend to sink down into the ground over the centuries. Thousands of years of ruins have been built over and over themselves, pushing ancient structures deep underground. All manner of creatures make their home among the dark spaces of the Undercity. Most people in Stormgate avoid going underground if they can help it.
The tunnels closest to the surface consist of the Drainworks and underground canals. These are used for storm drainage and sewer infrastructure, and are frequented by smugglers and criminals in order to traverse the city covertly. The Gondoliers are one notable faction that takes advantage of this area for transporting goods and information.
The City of the Dead extends down from an entire district at the surface. A twisting latticework of catacombs and tombs riddle the stone beneath, where Stormgate's countless dead are interred. Given the city's unsavory history of necromancy, the deeper reaches of the City of the Dead are mostly sealed off.
Deeper underground in scattered caverns, Undertown brings a sample of the Underdark to Stormgate. This slum is inhabited by myriad of underground races and creatures, many of whom have been exiled from the Underdark proper. Undertown is currently presided over by Mama Croon, a Drow Matriarch that acts as a sort of regional governor.