One of the largest caverns in the world has a maw large enough to stack cathedrals in.

In paradise, feral pigs have claimed an island all to themselves.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

One of the most remarkable caves in all of Iceland was created by a once raging river of lava.

This canyon is comprised of gorgeous waters and magnificent basalt columns.

An isolated, rusty phone booth commemorates Iceland's first international telegraph cable.

Surrounded by ash fields in Iceland, an impossibly verdant valley looks like something out of a fairytale.

This valley of hot springs and boiling mud pots is home to the record-holding "Geysir," which originated the English word.

A winding, blood-red tunnel of hardened lava navigable by foot.

Climb right into the heart of this colorful Iceland volcano.

Iceland's "Smoky Valley" is a beautiful river that is shrouded in a constant cover of steam clouds.

Walk in the footsteps of the gods and straddle two tectonic plates.

Strange rock formations tower above the river that snakes through this enchanting Icelandic canyon.

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

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

Medicinal spa created with the discharge from a geothermal energy plant.

Hotel designed by indie graphics artists.

A symbol for the opposition to nuclear power around the world.

A small Denmark town displays a pair of titanic jaw bones as their community symbol.

Nearly 40 miles and 1,000 years of limestone mines under Denmark.

Museum on earth sciences featuring a recreation of a famous curiosity cabinet.

This column celebrates Rome's victory over the Goths, and may be the oldest Roman monument in Istanbul.

This hidden gem is bathed in beautiful natural light.