Setting
"Here lies Old New York."
Most of the buildings resemble architecture from the 21st century. Technology is just as advanced as top side, but rarely seen since most can't afford it. Instead, it is common to see some out of date objects. There are no streets, just dirt roads littered with trash. There is also a unique aroma that fills one's nostrils at all times, nothing bad, but an air completely different from above. Sunlight rarely reaches these depths, which makes most of the people born here paler than normal, they are also a bit sun shy and can tolerate to cooler temperatures.
The Undercity is basically a built over Manhattan. Not being able to keep up with times, they were left behind. Ignored and forgotten, they forever remain in the shadows of the new and better city.