t was a hot Friday afternoon, with mounds of white clouds in a dazzling blue sky, and I was hiking along the gravel of the Chesapeake ---- Ohio canal trail in Western Maryland. To my right, down a steep, wooded embankment flowed the waters of the Potomac River. The river here, at a place called the Paw Paw Bends, twists and snakes in five horseshoe-shaped turnarounds within a six-mile stretch.To the left of the trail is a muddy ditch, spouting with wetlands plants. It’s all that’s left of the historic Chesapeake ---- Ohio Canal. The canal was built from 1836 to 1850 to carry coal and other goods from the Appalachian Mountains 184 miles down to Washington, D.C.I pass a stand of sycamores trees, and suddenly, up ahead, I see something dramatic: The gaping mouth of an ancient-looking tunnel, carved from rock to allow the canal and trail to pass through the mountains.This is the entrance to the Paw Paw Tunnel -- in my view, one of the most amazing places in Maryland for hikers and nature lovers to visit, and a gem in the chain of scenic wonders that line the C ---- O Canal Trail National Historic Park.