This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

Disks are rolled out onto the south lawn to absorb the impact of Marine One's wheels like giant coffee coasters.

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

Once home to seven different roller coasters, Glen Echo has undergone many transformations since its founding in 1891.

The tallest The Church of Jesus Christ of Latter-day Saints temple in the world soars above the Beltway.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

Maryland's first (and last standing) toll house on America's first federal road established the state as the "Gateway to the West."

The antique hotel still has a jail in the basement, though the cockfighting ring is long gone.

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

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

This general store is more than a century old and home to various oddities and antiques.

A monument marks the location of the first successful steamboat demonstration.

Wedged between two sets of row houses is an awkwardly located family graveyard.

Baltimore's classic clock tower was once topped by a rotating, 20-ton medicine bottle.

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

A faded sign from the 1920s remembers Baltimore's resistance toward banning alcohol.

A 200-year-old bar with a cheeky name claims to have served Edgar Allan Poe his final drink.

A blank headstone topped with a pile of pennies marks the final resting place of the infamous assassin.

A hidden alley in urban Baltimore is a haven for graffiti artists.

Fiji mermaids, mummified remains, and recreated cryptids are among the morbid oddities at this Maryland museum.

The man who first patented the Ouija board rests in peace beneath a headstone that playfully reflects that achievement.

The site where Poe "in great distress, and ... in need of immediate assistance" likely died.