Rob Cohen originally wanted Nicolas Cage to play Kit Latura. Universal execs felt he was more of a "character actor", and Sylvester Stallone was more commercially viable.
The tunnel sequences were filmed in Rome's Cinecitta Studios, on a set a third of a mile long. Cinecitta was chosen as the main studio set because of its enormous floodable sound stages.
The survivors climb the stairs in the sandhog bunker to a small room below a maintenance parking lot. Since there were 8 steps to the hatch under the booth, then 20 steps to the parking lot, the lot was about 30 feet above the tunnel. The lowest point of the Hudson River is 93 feet deep, plus 10 feet of bedrock on top of the tunnel, so the lot would be under 60 feet of water.
When the explosion reaches the mouth of the tunnel, the cars entering stop in straight lines. After Kit and his passengers get out of their car, completely different vehicles are scattered everywhere, and some have overturned.
When Eleanor is holding the pipe in George's mouth after he has been pinned under the truck, the sleeve of a bright pink wetsuit is visible under her shirt sleeve.