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

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 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?

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.

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?

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

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

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