Downtown Steam Tunnels
It may seem archaic, but many of Downtown's buildings are still heated with steam. It's all produced by the We Energies Valley Power Plant just south of Downtown. Those steaming brick stacks you pass on the I-94 bridge over the valley? That's the plant. The steam is carried to the Downtown buildings it heats through pipes underground, which you can see above ground in one spot: inside the arch over Canal St under the freeway. The system then proceeds underground -- including under the river -- to Marquette, the Milwaukee Art Museum, MSOE, and its many other customers.
The land that is now Havenwoods State Forest in the north of the city has a storied history that includes a stint as a County House of Corrections in the early 1900s, Army disciplinary barracks for prisoners of war, and a Nike Ajax missile site in the '50s. It's no wonder then that there were tunnels dug under the park. While many of these had steam pipes, but it's not known when or why a lot of the tunnels were built. In the Army base days, it's likely tunnels connected barracks and other buildings with the Commandant's building. Don't get any urbex ideas, though: any tunnels that haven't already been sealed or destroyed are filled with asbestos.