After several rounds of budget cuts, and some major structural problems the Big Dig was eventually shut down. They closed off the tunnels for safety and soon after various squatters moved in and made them their home.
Many rumors circulate about the true cause of the structural problems as the tunnels were in use for several years. At this point the truth may never be known.
The "central" location in Dig Town is the bazaar. This is an old abandoned subway station that sunk about a story during the quake. Suprisingly the area remained largely undamaged. This acts as an impromptu trading post and market for the inhabitants. It also is a convient meeting location given how vast and confusing Dig Town is.