I came across this one once before but kind of forgot about it. There is a manmade tunnel near the Delaware Water Gap and not far from the Pahaquarry Mine that apparently was driven while the government was going to damn the Delaware long ago but never did. There is an interesting articles giving directions to the lengthy tunnel at
http://www.nnjg.org/newsletters/speleothemes-2002-11.pdf. What is funny is how the cavers specifically say the No Trespassing sign in cavers' speak means, "Welcome, enter here!" Goddamn hypocrites!!
Unfortunately it is batgated but even as the article says, one can squeeze underneath the gate. It doesn't really say how much room there is but it might be worth checking out, even if it isn't an actual mine -- it is a manmade tunnel into the earth and is fairly long.