Construction on the Paw Paw Tunnel, part of the C&O Canal National Historical Park, started in 1836 when engineers decided to build the Canal right through, rather than around, an intervening mountain. The result was an engineering marvel lined with more than six million bricks passing 3/4 mile through the hill. Today, the tunnel is open to walkers, who can make the one-way trip in 20 minutes. This is not for the claustrophobic or those afraid of the dark (you must walk down a narrow towpath bounded by a guardrail, with the canal on one side and a sloping brick wall on the other). If you're up for it, pass through the massive brick arch into the darkness, then head for the light at the end of the tunnel. A flashlight is a must.