About 80 feet below you lies Indiana's longest tunnel. Over 4000 feet long, this tunnel allows for rail service to the west from the valley below. Construction began in 1870 at the cost of nearly $1 million.
Trains first began using the tunnel on Oct 9, 1882 and allowed several rail companies to provide service between Louisville and St. Louis. Construction of the 2.9 grade tunnel took 10 years to build, cost two men’s lives and also caused the bankruptcy of several construction companies. The tunnel is 15 feet wide, 24 feet high and up to 89 feet below the surface.
Near this location, inside the fenced area, is one of two 70-feet deep air shafts that allow exhaust to escape from the tunnel.
https://bridgehunter.com/in/floyd/duncan-tunnel/