This beautiful building in the heart of Manhattan's Financial District was empty for decades.

The prototype for all Nazi concentration camps.

An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

Home to items never before seen by the public.

Breathtaking and huge 430 year old maps of Italy.

Hear the quietest sound from across the dome.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

Go inside to really appreciate this 500-year-old technological masterpiece.

Elevated freight railway turned wildly successful urban park.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

Must form and function forever remain at odds? Not if the Florida Department of Transportation has its way.

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

This sprawling, formerly elite sporting complex in Amman, Jordan has become a local picnic spot.

Space-age minarets towering over Islamabad.

The most confusing clock in New York.

This exemplar of modernist architecture is meant to captivate and inspire students of the Cooper Union.

World's first "penitentiary," meant to be humane, drove men insane.

Largest Roman Catholic church in North America.

The site of the climactic scene from the classic horror film is now a historic landmark.

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

Over the centuries, this island paradise has done it all, from providing monks with ecclesiastical respite to sheltering an exiled Leon Trotsky.

Five sultans and their families rest in eternal splendor, teeming with overlooked beauty and drama.