Besides 40k that is.
>Your PCs are all regular, ordinary folk riding the subway late one night, trying to get home.
>One by one, each character fell asleep, missing not only their stop, but every stop.
>By the time they wake up, they discover that they and their train have been left abandoned.
>They know they have to fin their way upward, and they know they have to stick together.
>What they don't know is just how far down they really are, or what is down there with them.
What do your PCs find beneath the city streets, in the secret places deep underground?