Incredible labyrinth of man made caves.

2300-year-old Roman road connecting the empire.

Subterranean place of worship in Indonesia.

Massive sinkholes on a mountain in one of the most remote places in Venezuela.

Rock-cut cave complex filled with ancient Buddhist artwork.

Hydraulic systems in an ancient underground Iranian city.

Scattered throughout London are ancient remnants of the city's former bounding wall.

A 2000-year-old Roman amphitheater lies just below Guildhall Yard.

This complex of vertically-excavated Buddhist, Jain, and Hindu monolithic temples in India's Maharashtra providence illustrates a spirit of tolerance characteristic of ancient India.

Most-visited and most-photographed slot canyon in the American Southwest.

Eons ago, a rushing lava river formed this pristine passage through solid rock.

Rock-cut chamber at top of an outcroup, reached by a mountain stairway of forty steps.

A cave's rare beauty held a cavernous secret hidden underground.

Underground monastery and fortress.

A unique underground structure dating back to the 1st century BC.

This series of Scottish caves has no clear purpose other than baffling historians.

Figures of unknown origin carved into the surfaces of underground caves.

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

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

Wonderfully preserved Victorian factory, complete with a preserved steam powered engine house.

Enormous underground chamber built to detect 'ghost particles' of the universe.

These azure waters were thought to lead to the Mayan Underworld.

Learn about Iceland's hidden folk at this school dedicated to the study of elves.

This fantastical waterfall is hidden in a cave located in a cracked cliff.