Tunnels

This ancient tunnel is one of the most important examples of Roman hydraulic engineering in Spain.

This trail leads to a mine that was used as a sanctuary during the Great Fire of 1910.

The narrow passages will challenge even the bravest driver.

A labyrinthine underworld of tunnels and caverns hides beneath the city.

This recently restored crypt was once plagued by liquefying coffins and exploding caskets.

The site of one of the largest prison escapes of World War II.

The blowhole bursts spectacularly right next to the buildings in a medieval seaside town.

This old train trestle and collapsed tunnel once allowed the railroad to negotiate the steep slopes of the Colorado Rockies.

This subterranean passage reveals the city's mining history as well as its legendary terrors.

An abandoned, desolate train tunnel that was used during World War II.

This peculiar Italian elevator runs both horizontally and vertically.

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

Canada's oldest railway tunnel is illuminated in a rainbow of color as it passes under the streets of this small city.

The tombstone of an unfortunate explorer who disappeared in the Paris Catacombs.

This nondescript building hides an entrance to an underground nuclear tunnel system.

This Gothic castle-like portal is a testament to the amazing craftsmanship of the navvies who built the railway tunnel.

Abandoned for over a century, this chilling train tunnel is now a destination for ghost hunters.