A swimming hole in the historic industrial quarry that supplied the marble for the Baltimore Washington Monument.

Once part of a dam servicing an early 19th century grist mill, the tucked-way falls are a tricky-to-find urban oasis.

The stone marker that Mason and Dixon placed at the eastern end of the famous north-south border they surveyed.

Politicians, bureaucrats, and military officers gathered in this field near the Capitol to settle their disputes with blood.

A star fort along the C&O Canal in western Maryland that protected America in three wars.

Once America's tallest structure, now a Baltimore landmark.

This one-room cabin where the young colonel slept during the French and Indian War.

The whole world in one water tank.

This fort down the Potomac from Washington, D.C. was once the only defensive fort protecting the capital.

A remote horde of architectural treasures fit for Indiana Jones.

Built by the patriotic residents of Boonsboro in a single day.

Live data is transmitted from the Atlantic coast to instantly recreate the ocean waves in this fountain at the NOAA headquarters.

This bug-eyed water wheel uses the power of the Sun to clean up Baltimore Harbor.

This simple convenience store sits on the location where the Ouija board was named—and has a plaque to prove it.

After hundreds of years of settlement, this once-bustling fishing village sunk into the Chesapeake Bay.

The land is home to swimming ponies and a legendary 18th-century treasure.

Abandoned former poorhouse, bible college, and haunted house in an Ohio hamlet.

The tragic reality of a town riddled with rumors of haunts, chemical spills, and serial killers.

After covering almost 1.5 billion acres, the survey's still not finished.

The failure of one component toppled this entire bridge during rush hour.

Inside houses a vast collection of military aircraft and missiles spanning the ages.

One of the few places where you can explore a 1918 paddlewheel steamboat from stem to stern.

One of the last few period-correct, hand-operated canal locks in the country.

At the center of this cemetery stands a 2,000-year-old burial mound of the Adena culture.