In the late 1940's a system of underground tunnels were built below Dupont Circle in DC to alleviate traffic and provide a stop for the trolley system that existed at the time. The trolley system was abandoned by the city in the 1960's and the tunnels were sealed off from pedestrians.
Apart from a short-lived attempt at reviving the tunnels with the construction of an underground food court that never took off in the 90's, the tunnels remained closed for over 50 years and most people in DC have no idea of their existence.
More recently however, the DC Arts Coalition was granted use of a small section and uses it to display local art.