Sultan Mehmed II's siege of Constantinople enters its third week. Mehmed's artillery teams continue their relentless bombardment of Constantinople's walls. After his navy suffers a devastating defeat, Mehmed moves his ships overland to the Golden Horn in a daring, visionary feat. In the shadow of betrayal, Giustiniani attacks the Ottoman fleet.

Right now cosmic forces prowl the universe and threaten man's very existence. They're asteroids and comets; they've left their imprint on planet Earth, literally. Initially helping to build planets through violent collisions, during this fiery bombardment period they may have even seeded Earth with water and the building blocks for life. Since the turbulent formation of the solar system, these space rocks have continued to impact Earth. Some have been so violent that they've led to mass extinctions, including one that wiped out the dinosaur. New theories suggest that asteroid and comet dust harbour deadly viruses that may have triggered some of our worst pandemics. The possibility of future collisions remains a legitimate threat yet despite their dangers, asteroids and comets may hold vital natural resources, which could actually preserve

It sounds like a Hollywood blockbuster: a deadly asteroid is on a collision course with Earth. But in reality, it's only a matter of time before a giant space rock threatens to wipe out civilization. An asteroid took out the dinosaurs sixty-five million years ago. Are we next? This episode analyzes the threat and explores the many ways--from a nuclear bomb to ingenious new technology--that experts are proposing to stop Armageddon

For the past century, Russian history has also been the history of its security services. They were used by the Soviet state to crush dissent. Millions suffered at their hands. Mass executions, secret wars, spies capable of stealing the atomic bomb, poisoning scandals all add up to the most extraordinary and dangerous security network the world has ever known. But even today, the security network is arguably stronger than ever. This is the History of the KGB, told through its veterans and its victims.
Founded in 1917, Cheka was the predecessor organization of the KGB. Set up as a 'temporary' measure by Lenin, unknown number of Soviet citizens would die at the hands of the secret services as internal dissents in the 1920s. An inglorious chapter is the "Great Terror" phase, when millions of Soviet citizens were convicted and executed in mock trials under Stalin's rule in the 1930s. Outside the Soviet Union, during the WWII they were busy infiltrating German High Command, British Intelligence and America's Manhattan Project. The Cold War had begun in earnest.

As World War II drags on into another bitter winter the Allies become desperate to end the conflict. Winston Churchill turns to his formidable bomber force to break the deadlock.
The population of an eastern German city is caught in a devastating firestorm and harnessed by the Nazis; the controversy turns the Allies on themselves.

Beneath Yellowstone National Park, lies the biggest volcano on Earth. An eruption in the past was so big it plunged the earth into a volcanic winter that lasted years. A super-eruption would be more than millions of Hiroshima bombs going off all at once. It would be even worse than an asteroid impact: Entire cities lost beneath ash, people and animals crushed alive, power networks destroyed, sun dimmed across the globe, harvests failed, widespread famine. Could this nightmare really happen? We will use the latest scientific data to uncover the danger beneath us, as we see our planet like never before.