There is a tunnel on the light rail line (under the airport runways. As I argued to someone in another diary a few months ago, groundwater is no problem per se, a pressure-balanced tunnel builder machine will go through it, or you could build from the surface with diaphragm walls and ballast weight for hydrostatic equilibrum -- indeed most city tunnels are in groundwater. Varied strata, especially if it varies between hard rock and water-carrying sediment, or water-carrying layers deep in the route of mountain tunnels is what's problematic.

_{*Lunatic*, n.
One whose delusions are out of fashion.}