The Chicago Tunnel Company

As if Chicago didn't have enough stuff above and under the streets to make it fall in at some point - the multiple levels of Michigan Avenue and the subways - apparently there's a massive system of tunnels beneath the Loop, about 60 miles worth, that from 1901-1959 were a freight line to clear out ash from boilers in the basements of Chicago's new skyscrapers and bring coal to them. Goods and even people were sometimes transported along these lines. They were electrically powered via caternary cables. When coal was lesser used starting in the 1950s, and truck deliveries became cheaper, the system became run-down and then abandoned.

When originally created, the system was well-built. For example, when it crossed under the Chicago River, steel doors were installed to keep the water out were the tunnel roofs to ever collapse. Sadly, by the 1980s, almost everything, even most of the rail cars, had been removed by scrappers or destroyed by vandals. In 1992, the roof of the tunnel under the river did collapse and the system did flood as well as many basements that it connected to. Since then, the system has been pretty much impassible even to urban explorers. In the 1990s, Commonwealth Edison renovated some of the tunnels to carry high-voltage electrical lines. However, most of the system is still abandoned.

See the full site here.