1500 miles worth of tunnel beneath Odessa in Ukraine:
Not fully mapped but this is what they do know:
Really disturbing bit:
Bunch of friends have a new years party in there in 2005. One of the girls gets lost, maybe took a wrong turning somewhere when she was pissed and couldn't find her way back. Her body is discovered a few months later 3 miles away from where they'd entered. Graphic photo of discovery of body if you're that way inclined: http://whatsupkuwait.files.wordpress.com/2010/09/odessa-catacombs-10069-large_slideshow.jpg
For some reason the government didn't remove her body for proper burial until 2007.
My sleep has been disturbed imagining what she went through when she realised she was completely lost in the pitch blackness of the tunnels unable to find an exit. Assuming she died of dehydration that's at least three days of desperate wandering.