Mystery At Hoover Dam

I'm fascinated by things and places that are not open to the public.

Can somebody tell me how these vehicles got to the bottom of Hoover Dam? There appears to be a tunnel at the southwest end of the powerhouse or whatever this is at the base of the dam, but ... where would it come out? Or is there a vehicle elevator in the dam itself for the dam workers' vehicles? I've spent the last hour trying to solve this mystery and I'm just as confused as I was the first time I saw them parked there.

Here's some screen captures from Google Maps Satellite View to show what I'm looking at.

The mystery has been solved, after another hour of research. There is a tunnel ... it comes out about half a mile downstream of the dam and the "Lower Portal Road" winds its way back up toward the dam, meeting with the upper road about 900' west of the security checkpoint.