|Cumberland Gap Tunnel (Tennessee Portal), Cumberland Gap, Tenn.|
Vehicular travel now travels under the mountain through the Cumberland Gap Tunnel. Begun in 1991 and opened in 1996, there are two two-lane tubes with each traveling 4,600 feet. During construction, subterranean mountain streams would have pumped hundreds of gallon every minute into the tubes. As a result, the tubes are lined with thick PVC piping. [*] The total project to construct the tunnel ran $280 million (the project was about 2x budget).
Interestingly, the Cumberland Gap Tunnel is one of only two tunnels in the United States that traverse a state line (the other is on I-77 between W. Va. and Va.). Although the old US 25E meandered also through Virginia, the tunnels do not (though they are mere yards away). [* - great link on planning, construction details]
Note: Yes. The photo was taken of the Tennessee portal, but the Kentucky portal looks about the same. And the latter is in Bell County. So there.