When a perfect triangle is found in the middle of a unpopulated portion of Australia, experts begin to wonder whether or not the design is man made.

Satellite photos show a strange circle beneath the ice of an ancient lake. Further analysis shows that these mysterious rings appear each year. What are they?

A satellite captures an image that uncovers the entrance to an underground complex. What is being built there?

Scientists are baffled when a seemingly huge tsunami headed straight for Hong Kong suddenly disappears in the water.

Sandy Island seems to have disappeared, and scientists aren't quite sure why. Was there some sort of elaborate government experiment?

Satellite images capture a distinct glowing object beneath the ocean's surface.

Satellite images reveal a strange hole in Antarctica. After scientists rule out whirlpools and sinkholes, they have to look back in time to find the answer.

A commercial satellite imager spots something strange in the middle of a vast desert. It looks like an airport runway, but there are no planes. No government claims ownership of this strange place.

Satellite images reveal enormous spirals in Peru. Some researchers say they are too perfect and too huge to be man made.

Ghost lakes have existed for millions of years, but they just recently started showing themselves. Geophysicist Helen Fricker explains how scientists are moving forward with this discovery.

For years the hottest place on Earth ever recorded was a city in Libya. When researchers took a second look at this information, they realized the measurement was wrong.

When you think of deserts, you don't typically think of bright, jewel tones. Why are these massive pools of color here?

Who drew this over 2 mile long humanoid figure into the Australian terrain?

Satellite images revealed hundreds of black holes in the middle of the desert near ancient burial grounds. Is grave robbing still a thing in modern day or were these holes caused by something else?

Satellites spotted a strange, long streak across the desert in Bolivia. At first, scientists thought it may be a skid mark from a meteor.

This isn’t an optical illusion. This lake on Middle Island is actually pink! You can thank the algae in the lake for the gorgeous color.

Even stranger, it disappeared less than a year later.

This romantic road carved into a sheer rock face in the Rhone Valley in France was an engineering marvel that was once a wild success. What caused it to be abandoned today?

Built in the 1940’s the Goat Canyon Trestle Bridge was the tallest wooden railway system in the world.

Two miles inside a mountain in Croatia lies the remains of a massive, secret Cold War airbase built to withstand a nuclear war.

Now abandoned, this machine once could scan thousands of kilometers.

Explore the mystery behind why a huge gun was built and abandoned.

Hidden in woods outside Moscow is a Cold War weapon that harnessed electricity into a thunderbolt, designed to protect the city.

In the east China sea, there is an abandoned piece of land. Check out why this island represents one of the greatest engineering achievements of the 20th century.

Located in California, Plank Road is one of the most extraordinary engineering achievements of the early 20th century.

Philip Grossman finds firefighting gear under a hospital in Chernobyl, sending his Geiger counter climbing. He believes burying the gear was the Soviet’s attempt to hide the scope of the disaster.

Philip Grossman goes inside the plant near the massive radio tower he finds maps of the U.S and pictures of mushroom clouds. Was the Duga for protecting Chernobyl from incoming missiles or part of a nuclear offensive?

Survivors who worked at nearby factory report production areas they weren’t allowed to visit. Philip visits and finds the basement full of lab after lab of gas masks, radioactive testing materials and sand.

Since launching in 2009, the Kepler program has detected hundreds of previously unknown planets. How many of them are Earth-like, and what does that mean for the probability of intelligent life beyond our solar system?

Author Nick Sagan hasn't seen his godfather, Frank Drake, in over 30 years. We capture their reunion - and find out how Drake became friends with Nick's famous father, Carl Sagan, in this exclusive clip.

Author Nick Sagan hasn't seen his godfather, Frank Drake, in over 30 years. We capture their reunion - and find out how Drake became friends with Nick's famous father, Carl Sagan, in this exclusive clip.

What would receiving a message from an alien civilization tell us about human evolution, and about surviving the most serious problems facing our planet?