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.

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?

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.

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?

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

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.

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

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

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

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

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

Since the 1960s, NASA has been keeping an eye on man-made objects in space. Here, engineers are replicating the devastating impact of space debris. 'Alien Encounters' airs Tuesdays at 10/9c.

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?

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.

It's possible that life on earth began off-planet and spread here.

If the chirality of aliens is different than ours, it couldn't interact with us, and therefore would be non threatening.

It's likely that an advanced civilization would combine the advantages of a machine with the advantages of biological systems.

An alien spacecraft might not rely solely on sunlight to power its trajectory. Find out what other energy technology aliens could use to travel through the universe.

Will ETs look like us? Will they resemble giant insects? Scientists debate how various environments could have created life forms beyond our wildest imagination.

John Smith and his cousin Billy have a mysterious encounter while driving on a remote highway in the Yukon. Was it a UFO?

It's 1967, the Cold War is on and there's a glowing orange light hovering over a U.S. Air Force base.

Reports of a downed aircraft bring a volunteer firefighter to a remote wooded area.

The Hale - Bopp comet was overhead but a strange formation of lights in the sky didn't appear to be natural.

A Japan Air Lines pilot spots a mysterious object on his radar and sees bright lights outside the window of his aircraft.

A police officer spots a strange light in the sky in Minnesota. Things only get stranger after he crashes right into it... or it crashes into him.