The Caldecott Tunnel cuts through the East Bay Hills from Oakland to Lafayette, taking Highway 24 with it.  It consists of three bores for traffic to pass through and an extensive ventilation system that keeps the air in the tunnels breathable. Here are a couple of views of the intake vents housed above the west entrance:

The tour group entered through a door at the ground level and then climbed up to the catwalk you can see on the right in the picture below.  From there we entered the room where the Giant Fan Motors lived.  I don't have a picture of them ... yet.