http://www.abandonedturnpike.com/
I went to this place about one week ago, and I must say it is incredible. Bring a mountain bike or a hybrid at the very lease as the riding is not very gentle - the roads are in pretty horrible disrepair.
Basically, this was one of the first turnpike expressways in the US. Traffic outgrew the one lane tunnels, and so the turnpike commission had to close off a pretty significant portion of the turnpike. Whats left is a four lane highway which runs about 13 miles end to end. This includes two tunnels, one stretching 0.7mi, and another stretching 1.6 mi (according to my cyclocomputer). The tunnels were NUTS. The first one you can see end to end so it isn't that bad. The second, well, its pitch black inside. We stopped midway through and all you could see was darkness, and all you could hear was wind and the dripping of water. Bring flashlights!
Its a rather smoothe ride except for the rough terrain, and the land was purchased to be used exclusively for cycling. We met a ton of cool people.
Its great if you like exploring because some of the tunnel maintainance rooms and stuff can be accessed. Definately worth the trip!
If any of you have been there, what were your experiences?