A strange alien shrine on a mountain peak, a bridge built in the Alaskan wilderness, and an American castle with a dangerous secret are mysterious deserted engineering projects. New discoveries reveal how they were built and why they were abandoned.

An American ghost town in the Amazon jungle, a secret Nazi base, and a deadly concrete fortress 866 miles long are mysterious deserted engineering projects, and new discoveries reveal how they were built and why they were abandoned.

A terrifying sea fortress, a highway in the Amazon that suddenly vanishes, a mysterious structure in the shadow of the Statue of Liberty are deserted engineering projects, and new discoveries reveal how they were built and why they were abandoned.

Eerie ghost ships stranded in an alien desert, sinister ruins known as the Dragon's Teeth, and a ghostly complex with a bloody past are deserted engineering projects, and new discoveries reveal how they were built and why they were abandoned.

A ghost ship thousands of miles away from where anyone expected, a bridge to nowhere in Florida's Hurricane Alley, and a haunting mountain complex with a bloody secret are deserted engineering projects. New discoveries reveal why they were abandoned.

Six breathtaking images, six unfathomable mysteries: from a gigantic tsunami in the air that never hits land, to a secret underground base in China, to an image from space that may reveal the location of the Garden of Eden.

Weird barren rings in the Namib Desert, an island the size of Manhattan that vanishes without trace and a lost city in the heart of the Amazonian jungle. What on Earth? Investigates six more mysteries captured from space.

First seen from space, the Eye of the Sahara is a beautiful 25-mile structure, but how it formed remains a mystery. And are extraordinary clouds over an island in the Indian Ocean evidence of a terrifying new weather weapon?

NASA astronaut Mike Barratt describes photographing a strange ring in the ice on the world’s oldest and most mysterious lake from the International Space Station. More rings on Lake Baikal have been seen since, baffling scientists.

A possible smoke screen over North Korea worries military analysts while an amateur archaeologist makes an extraordinary discovery with a satellite image -- has this image revealed not one but two forgotten pyramids in Egypt?

Images of earth from space reveal an intimate portrait of human activity. Cities glow bright while the wilderness remains dark. Why then is the Australian outback, lit up like a series of vast cities?

What would happen if we got a message from space? How would we react? Scientists and sci-fi writers, with cooperation from SETI and Carl Sagan's son, Nick, unravel the impact of this world changing event, forming an intriguing hypothetical scenario

How will humans react when an alien spacecraft is on its way to Earth? Will humans learn from aliens, or become colonial subjects? Scientists and sci-fi writers, with cooperation from SETI and Carl Sagan's son, Nick, unravel the impact.

The Arrival has ended. A swarm of small spacecraft depart their huge mothership, and hover in our atmosphere. The ships open their hatches, and deposit thousands of mysterious pods on the earth’s surface. Are they a message, a gift or a weapon?

A mothership the size of Texas is hovering over Earth and we're tired of waiting. We send a rover to explore, and what we find makes us question the definition of life. On Earth, the pods multiply, making contact with them unavoidable.

Go beneath the surface to understand forest fires, car exhaust, and the space station.