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

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

A labyrinthine network of chambers with a storied past hides below Edinburgh's South Bridge.

This small museum in a 17th-century building honors three iconic Scottish writers.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

A treasure trove of Renaissance-era funerary art in a Roman church built on top of Nero's grave.

A vast collection of rare books, manuscripts & extraordinary objects illustrating the history of medicine.

Yale University's home for rare works, including the mysterious Voynich Manuscript.

The headquarters of the famous Yale society.

Museum dedicated to literary greats of the Romantic Period is also the home in which Keats met his untimely end.

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

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

The final resting place of the poets Shelley and Keats.

This remarkably well-preserved Egyptian plinth is pretty well traveled for a giant piece of stone.

A marvel of Byzantine engineering under the streets of Istanbul.

A marble Lucifer hides behind the pulpit of this Gothic cathedral.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

A Nesting Doll of Churches.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

Step back in time and witness the fall of Constantinople.

Ancient serpentine sacrifice holder that has stood the test of time.

A 16th-century horror show built in a lovely Italian garden.