Golden Gate!
I hadn't thought of it until Tom B. mentioned the Golden Gate earlier. The piers sure look as if it would take a ship loaded with conventional explosives to do anything. That might be tough to do.
I wonder what would happen if a truck loaded with explosives totally breached the roadbed, releasing tension on the cables. (Actually, getting caught on that bridge during an earthquake is enough to make someone not used to "terra firma" not being "firma" nervous.)
Dunno.
Arf