Some of these "tunnels" are prefabbed on shore and towed and sunk into place. Examples of the Hampton Roads bridge-tunnel sections are about halfway through this vid:
Wasn't Cher* enough?
*(see the If I Could Turn Back Time video if you are unsure of the reference.)
I know the reference....... But these girls were great.. They were dressed like they were from a C & C game. I'll find a video