The tunnel boring in Seattle began right at the edge of downtown, went under a highway and a monumental retaining wall (They covered that wall with surveying targets and monitored it the entire time), then pretty deep under a residential neighbhorhood and a canal.

I think the only time it's moving close to high-rises again is over by the University.

Edit: I believe they went so deep because of the volume of glacial deposits in this area. Otherwise they would have been going through gravel. That made building the stations a bit of a pain (the Beacon Hill station still has chronic problems with water. It smelled like a moldy towel for a long time).