A wonder of natural limestone formations etched by millions of years of natural interaction between rainfall and rock and air.

An enormous, 30-story-deep hole in the middle of Guatemala City.

Remote Honduran Islands with a colorful history.

Fantastic lake on the heart of a fault line.

A marvel of Byzantine engineering under the streets of Istanbul.

Display that gathers the basic elements of the universe in one convenient location.

An abandoned whaling station on an active volcano offers hot springs in Antarctica, surrounded by ruined giants of industry.

One of the world's southernmost bars.

Hidden beneath the streets of London is a secret subway that only carried mail.

A lake in the middle of the tundra of Quebec is said to be the purest freshwater lake on Earth.

The bears of Bern have been a city highlight for over 500 years.

Caves famous for the dragon-slaying saint who lived there.

A historic observatory precariously perched at the "Top of Europe."

Iron Age grave stones and Viking burials, a short drive from Gothenburg.

The Los Angeles flood disaster that took 400 lives and ended the career of William Mulholland.

Millions of glowing microscopic organisms light up the waters of this luminous lagoon.

Do these monsters look familiar? The gargoyles on this church are all from pop culture.

One of the final concrete ships built before World War II is now a crumbling breakwater.

Come crisp December nights, this fairly plain neighborhood becomes a holiday dream world.

An island in the middle of a lake, in the middle of a volcano, in the middle of a bigger lake, in the middle of a bigger island, in an archipelago, in the Pacific Ocean.

An Australian lake whose pink hue defies scientific explanation.

An ethereal glowing blue ocean cave in Croatia.

An alien landscape of chalk-rock in the Egyptian desert.

Even King Tut was intrigued by the glass of the Great Sand Sea.