Sloth rehabilitation center in the jungle of Costa Rica.

This neon blue river gets its color from a unique mix of bacteria and volcano minerals.

Sparkling, calcite-covered skeletons of children who may have been sacrificed to appease an angry god.

Every equinox this Maya pyramid puts on a spooky ancient light show.

This portion of a lovely Belize beach is infested with (harmless) sharks and rays.

A girls' boarding school inspired by the Chicago World's Fair, once abandoned, now restored to strange and scenic glory.

Every drop of D.C. tap water flows through this old waterworks.

A secretive collection featuring objects that were designed specifically for U.S. intelligence operations.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

The world's oldest continuously operating airport was founded by the Wright Brothers.

A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

The most accurate timepiece in the world.

Giftshop catering to cryptozoologists and anyone with a sense of humor.

Before Metro, Washington had a robust streetcar network—and you see the remains of this infrastructure if you know where to look.

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

D.C. once had its own tiny rival to the New York Stock Exchange.

Where the U.S. government kept its actual treasure, before Fort Knox.

D.C.'s first central library was born out of a chance encounter with the philanthropist whose name it bears.

Home to items never before seen by the public.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

Look closely and you’ll notice that the color changes a third of the way up the tower.

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

Need a hardcopy of the 50-title Code of Federal Regulations? This is the place.

The Pentagon is so large that it was planned like a city, complete with internal highway infrastructure.