Backbone Rock tunnel near Shady Valley, Tennessee. Known as the shortest tunnel in the world, it was blasted through to make way for the Beaver Dam Railroad in 1901. This part of the Holston Mountain outcrop is 75 feet high and 22 feet thick. The railroad continued operation until 1918, when the lumber and manganese ore played out. In the early 1930s, the tunnel was widened for two lane Highway 133 to permit car travel between Damascus, Virginia and Shady Valley, Tennessee.