This domed temple was built in a hidden cave mouth as a refuge for sinners seeking absolution.

This Irish show cave is said to have been the site of a Viking massacre that claimed 1,000 lives.

This French anatomy museum greets guests with a skinless man, mid-shovel.

This historic covered bridge contains a series of macabre paintings that communicate one thing: everyone dies.

Architectural monsters are devouring this palace's every door and window.

The yawning maw of this pagan visage is said to bite off the hands of liars.

Perhaps the most beautiful of preserved saints, with a little help from Paris.

The beautiful baroque library of the Hapsburg empire.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

The glowing blue Nymphaeum of Emperor Tiberius.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

Stalactites and stalagmites in a beautifully lit cave.

This remote castle was built to be a wealthy businessman's palace of vice but is now simply a historic vacation cottage.

The second largest surviving chained library is housed in this 12th century church.

Rare collection of medieval books under lock and key.

One of the oldest libraries in England, and one of the last of the great medieval chained collections.

A gorgeous medieval library where the books are kept in chains.

This small collection of rare books was uniquely locked up for safe keeping.

Unchanged for centuries, this medieval library keeps its books in chains.

Built by the patron saint of illusionists this Elizabethan manor house is riddled with secret cubbies for hiding priests.

The fairy tale castle of the "fairy-tale king."

This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

Trick water gardens from 1619 featuring a 200 piece mechanical theater powered by water.