This is the last tunnel on what is now the mostly abandoned N&S line coming from Knoxville to Cumberland Gap.  This is Greer tunnel, close to a feature called “Greer Lake”  (not seeing a Greer Lake, folks) and New Tazewell.  This portal, the northwest, is at 36.423103, -83.591183.  The tunnel, soggy and drippy, seems to be about 350′ long.   The history of this railroad and the changes in its trackage provide interesting reading.