The Cincinnati Subway is a set of incomplete, derelict tunnels and stations for a rapid transit system beneath the streets of Cincinnati, Ohio. Although it is only a little over 2 miles in length, it is the largest abandoned subway tunnel system in the United States. Construction began in the early 1900s as an upgrade to the Cincinnati streetcar system, but was abandoned due to escalating costs, the collapse of funding amidst political bickering, and the Great Depression during the 1920s and 1930s.