This four-story steel sphere in Maryland was used to test biological weapons.

A Civil War Museum with some notable medical antiques.

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

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

The national park was once a plantation estate.

A rotating cast of guys and dolls in front of a Washington, D.C. building.

At 207 feet, one of the world's longest continuous escalators.

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

A hidden passageway now marks the site of an assassination attempt on Ronald Reagan that some say broke a 140-year-old curse.

An astounding collection of medical specimens, including bits of President Lincoln.

Brain collection and database that narrows the gap between man and animal.

The headquarters of the Audubon Naturalist Society is a small slice of heaven for nature lovers.

She stares out across six lanes of traffic, clutching a musket and infant with determination.

The whole world in one water tank.

This ruined quarry once produced the distinctive red sandstone used for construction projects in Washington, D.C.

Built in 1815, the stabilized ruins of the grist mill and sawmill are now gated, but interpretive displays on the features and functions of the mill preserve its history.