An 18th-century Spanish watchtower made entirely of seashell and concrete.

St. Augustine's record-setting narrow street was designed to protect against pirates.

One of the most influential presidents in U.S. history wanted only this plain, elegant monument as his lasting memorial.

Once America’s most beautiful army base, this abandoned military fort on the coast of Monterey Bay has been left to the elements.

This "life-size" depiction of Mickey Mouse as a half-lobster is both whimsical and a bit creepy, standing guard over the Boston waterfront.

Etched into a dry lake bed, this massive directional indicator is accurate without magnets.

Low-frequency noise compared to an idling train can only be heard by some of this industrial town's residents.

The most accurate timepiece in the world.

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

Decades before the scandal, this staircase on the river was a literal "water gate."

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

A dark patch of the ceiling at Grand Central Terminal which was not restored is still stained brown by tobacco.

This curved Manhattan building was built to house the opulence of John D. Rockefeller's oil empire.

This whimsical group of statues is a favorite of children who love to climb all over Lewis Carroll's beloved characters.

This monument on Breed's Hill proves that one of the most famous battles of the Revolutionary War is misnamed.

A forgotten sea village stands silent after hurricanes deemed it obsolete.

South Carolina's last remaining slavery auction house is now a museum devoted to its own tragic history.

In its day this historic fort held 100 cannons pointed in almost every direction to protect New York.

This once-palatial plantation may have been the site where a draft of the Declaration of Independence was written.

A cave on the Maryland border where civilians took refuge during the bloodiest battle of the Civil War.

Soldiers were buried next to Lee's house in the center of Arlington Cemetery to dissuade the general from reclaiming his property after the war.

The remains of this visionary community can still be found much as it was a century ago.

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

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