Connecticut's shot tower is near the end of its life.

This unassuming chapel in Portugal plays host to a ghoulish collection of bones.

Iron Age runes and ruins hide in plain sight along this Portuguese beach.

A neolithic tomb converted into a tiny chapel.

A perfectly circular lake is formed by the crater of an ancient underwater volcano.

"Portugal's Stonehenge" is made up of nut-like stones aligned with the heavens.

Elaborately carved to showcase the glorious wealth that the age of exploration brought to the Portuguese empire.

A small Portuguese cottage formed between boulders in the countryside.

The medieval castle built as headquarters for the Pope's secretive Knights Templar.

This small chapel is built of human bones and decorated with a golden skeleton.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

Edgar Allan Poe's old dorm is now a shrine to the author's legacy.

Once the home of a hermetic monk, this cave now houses a small church.

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

Bizarre Cavern.

It's not hard to see why the historic Peabody Conservatory of Music's library has been described as a "cathedral of books."

This leafy estate is worth $18 million and is so grand it has two mailing addresses.

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

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

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

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.