Soon, you'll be able to add tunnels to your urban creations in Cities: Skylines [official site]. Even before the game had been released, Colossal Order had told me that tunnels were the one big feature they had been hoping to include at launch. I think the eventual outcome was the best possible scenario - the game was lovely at launch, mods instantly started to make it even lovelier, and now the tunnels are arriving in the form of a free update later this week.
I don't know what frightens me more - the inevitable video of an engineering mastermind's metropolis with perfect traffic flow enhanced by an entire underground transportation layer, or the inevitable video in which somebody manages to inexplicably divert every hearse into a tunnel that has no exit-point.
I've been working on a recreation of Salford and Manchester - two cities in one! - and while I can't think of any particularly important tunnels to include, I might pop one under Whitworth Park just for kicks.