It's closer to an underground version of an aqueduct than to a normal water main, a long-distance tunnel being bored to bring water from upstate reservoirs: http://en.wikipedia.org/wiki/New_York_City_Water_Tunnel_No._...

I'm not 100% sure why it's that deep (and can't seem to find a solid explanation). What I can glean suggests that it's partly due to wanting it to be in more geologically stable bedrock that passes below any bodies of water and geological aberrations in the area. And partly so that it can be fed by gravity along a continual downwards slope, which causes it to be fairly deep by the end (then at the end the tunnel narrows, increasing pressure sufficiently to push water up a vertical shaft to the surface distribution network).

I'm guessing they probably got there before everything on top was built, but how do they get down there now for repairs and such?

From Wikipedia:

"[New York City Water Tunnel No. 3] was authorized in 1954. A third tunnel was needed so tunnel one and tunnel two could be closed for repairs. Stage One was begun in 1970 and completed in 1993 and put into service in 1998."[1]

This seems to imply that the later stages of the old two tunnels have never seen repairs.

[1] https://en.wikipedia.org/wiki/New_York_City_Water_Tunnel_No....

They don't. That's why they are building a third water tunnel. For backup, and so they can do repairs on the other two.

Very hard, apparently, as jontas pointed out: http://www.nytimes.com/2008/11/23/nyregion/23tunnel.html?pag...

No - they're still building out these tunnels.

My uncle worked briefly on the project in the 60's but its very much active

http://www.youtube.com/watch?v=5gfSEjMtAoc

The water has to get in somewhere, and come out somewhere, so you go to those places and put a robot vehicle in.