Three sinkholes filled with warm, crystal-clear water reached by a horse-driven cart on an old train track through the jungle.

The ruins of a medieval monastery built on a perilous slope.

The ruins of a medieval village that was abandoned after a 19th-century inferno.

A fascinating assortment of cave dwellings, medieval ruins, and sarcophagi are being devoured by the forest.

This ancient altar emerges from the thick forests to reveal the mysteries of Etruscan sacrificial rituals.

Monks turned this mountain into a temple by digging grottoes, tunnels, and a huge pagoda-like hall.

A peculiar trapezoidal building covered with wonderful humorous reliefs.

Trek into the depths of the cave where 17,000-year-old paintings exist alongside 400-year-old graffiti.

A crumbling cliffside ruin hidden in the woods.

A jungle in the heart of São Paulo reminds visitors of what Brazil looked like untouched by man.

A stunning fountain depicting a mythological she-goat carved directly into the rocks on the grounds of a Renaissance palace.

The only theatre in the world constructed specifically for sound movement.

An artsy hidden staircase leads to breathtaking views of San Francisco.

Gold Rush ship-turned-hotel buried underneath San Francisco's financial district.

Golden Gate Park's historic collection of plants.

Tiki luxury on Nob Hill.

An ingenious solution to a tricky engineering problem, this spiraling bridge takes drivers for a dizzying spin.

One of the last of the great 1920s movie palaces is a Floridian Greco-Spanish-Persian dreamscape.

A patchwork of ruins from various civilizations in one of the world's oldest continually inhabited cities.

These tiny statues form a terracotta army from China's Han dynasty.

Whimsical sculptures and caves chiseled by nature over the millennia.

One of Yemen’s oldest and most beautiful mosques was named for the incredible, historical queen buried inside it.

Volcanic rocks ground down into sand make this Spanish beach as black as night.

A gorgeous stone viaduct crosses over a bucolic valley.