Massive crater in central Oregon.

The longest continuous lava tube in Oregon.

A series of waterfalls and pools that flow through the 'Ohe'o Gulch and into the ocean.

This legendary urban waterfall creates a dazzling display of rainbows.

A unique set of caves that spits waves back out into the ocean.

Submerged lava formations create incredible waterworks.

These picturesque Hawaiian falls are home to a rare species of fish that uses a special sucker to climb up cliffs.

This unusual lava tube spouts seawater during high surf.

A once-glowing grotto at the bottom of Waikapalae Wet Cave.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

In the basement of the U.S. Capitol Building is a hidden storage room full of ornate floor tiles leftover from the 1850s.

There just so happens to be a court even higher than the U.S Supreme Court: a basketball court in the building itself.

An 18-mile-long tunnel for sewage waste will soon wind within the bowels of Washington, D.C.

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

Obscure Freemasons still live in D.C.’s largest private residence.

An Industrial Revolution-era public work that purified water using nothing but sand.

A grotesque Fiji merman dwells within this traditional Dutch-style building.

A statue of the English poet for whom this Delaware town is named is ritually dressed in seasonally appropriate clothing.

One of the oldest surviving Swedish-American homes in the United States.

This local gem houses the largest collection of pre-Raphaelite paintings outside of the U.K.

This splendid Versailles-inspired mansion has the largest French-style formal gardens in North America.

A blend of art, antiquities, and Americana fill this museum, surrounded by whimsical, enchanting gardens.

This sprawling farm was once a Cold War-era hideaway for the nation's top diplomats.

A double agent hid information for the Soviet Union beneath this bridge for more than 20 years.