How exactly are the NYC underwater tunnels constructed?

Question

Every time I go into one of the tunnels I question how it is that they are deep enough under the river to really work? That is, it never feels like we've gone deep enough underground and I always wonder how the big cruise ships don't scrape the tunnel with their underbellies. Are the tunnels built BELOW the river floor or above it? I guess they MUST be deep enough since they obviously function. I just can't visualize it especially when you drive into one and it feels like you are still relatively even with land.

Thanks

Answer 1

http://www.nycsubway.org/articles/steinwaytunnels.html
This website tells all about it.