Tunnels

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

This long, dark tunnel is open to hikers, but those without lights are known to get confused even on the straight path.

An underground magnet for ever-changing street art.

The first full-scale tunnel built in America is short but scenic.

An exquisite tunnel of cascading flowers.

The railroad tunnel nicknamed the "Bloody Pit" for taking the lives of hundreds during its construction.

A well-preserved network of cavernous tunnels have spent silent centuries removing Toronto's waste.

A tunnel dug through the side of a mountain by hand provides convenient yet death-defying access to a small village in China.

A historic fort commemorating the United States' naval history.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

Vast network of air raid tunnels beneath Lombardy's capital.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

A hands-off space for graffiti artists to hone and display their skills.

A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

Known for the tunnel that leads visitors to the water.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

Extremely long cable tunnel buried under the river.

Longest canal tunnel in the world, abandoned in the 1960s.