This neon blue river gets its color from a unique mix of bacteria and volcano minerals.

Natural saunas fed by volcanic heat.

Hidden deep in the Guatemalan jungle is a turquoise paradise of natural pools, caves, and waterfalls.

Volcanically heated hot springs.

The ruins of an ancient Mayan city, beautifully preserved and only accessible by boat.

An elaborate Mayan city shrouded in alien conspiracy revealed the tomb of Pacal the Great.

Part bookstore, part café, and part forest.

A walk through the historic city center, under the streets of Seattle.

A busy cafe better known for board games than food.

Discovered after draining a local pond, these ancient man-made caves are still a complete mystery.

Home to nearly a third of the world's mud volcanoes, Azerbaijan is home to a messy, bubbling, and sometimes explosive landscape.

Right up until the 20th century this network of man-made caves was the biggest village in Eastern Armenia.

The second largest in the world, this massive underground void has its own subway.

Fortified homes of highland warriors that are still in use today.

A miniature train-ride through Slovenia's largest cave system.

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

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

Roman ruins, natural methane fires and a lovely beach.

Mysterious hillside with a continuously burning natural fire.

Mount Nyiragongo has erupted at least 34 times since 1882 and is home to the world's largest lava lake.

A fiery crater has been burning in the Karakum Desert since 1971.

Ethiopia's "smoking mountain" is in one of the hottest regions on the planet and is home to two lava lakes.

Houses are tucked between, on, and underneath giant boulders.

Africa's Venice on stilts.