The Paw Paw Tunnel was blasted through a mountain in the 19th century so that the C&O Canal could take a more direct route.  When you consider that the tools of choice were picks, shovels, mules, and dynamite, it is an amazing engineering feat.  The tunnel itself is long enough that you might want a flashlight in the middle of it, although the towpath is well-maintained.  There are some ruins of the workers' village on top of the mountain, and it's worth the hike back over to see those and the absolutely amazing views of the Potomac River and surrounding mountains. 

There are picnic tables and port-a-potties available, it's a nice place for a picnic.