Discover Cincinnati's underground (in more ways than one)
Cincinnati is a REALLY old city; incorporated in 1819, it was the first major city founded after the revolution, and a hotbed of activity on the Underground Railroad as the first stop for slaves crossing the Ohio River. Thus, you can (and should) visit the Freedom Center, also known as the National Underground Railroad Freedom Center, to learn about abolitionism in the past as well as the present.
Cincinnati’s heritage isn’t just metaphorically underground, though: In the early 1900s, citizens petitioned the city to build an underground subway to ease travel through its center, using the drained Erie Canal as framework. The subway was never finished; instead, 2.2 miles of tunnels were completed and left dormant since 1928, becoming a favorite of urban explorers and ruin-porn photographers alike later on. You can take an official tour and see it for yourself through the Cincinnati Heritage Program.