What sort of alien civilizations might exist in the vastness of space? Terra is the fictional world imagined in Episode 4, a planet nine billion years old, twice as old as Earth. Old enough that a truly advanced intelligence could evolve. It was once a fertile world, now it is barren. But life can still thrive here in artificial domes. Over time, they've evolved not to need their bodies. They exist only as neural tissue. They never age, they never die. They're monitored and maintained by robots.
If alien civilizations are statistically so likely, why haven't astronomers found any sign of them? Where is everyone? Every time we look at an individual star, that's like dropping a bucket in the ocean. We're going to have to look at a lot of stars and to search through a lot of data until we find the clue that leads us to another civilization.

It is the greatest technological challenge ever undertaken by humankind. The quest to produce the ultimate energy solution, Fusion, a reliable source of clean power. What if we could create here on earth, our very own version of a star, a machine that makes power from nuclear fuel? But so immense are the challenges, the goal of a power-producing fusion reactor has so far remained elusive and out of reach... until now.

Cities are growing at a faster rate than any other habitat on Earth. They may seem an unlikely place for animals to thrive, but they can be a world of surprising opportunity. Leopards prowl the streets of Mumbai, peregrine falcons hunt amongst New York's skyscrapers, and a million starlings perform spectacular aerial dances over Rome. In Jodhpur, langurs are revered as religious deities and in Harar, locals live in harmony with wild hyenas. Many animals, however, struggle to cope in the urban jungle. As the architects of this environment, can humans choose to build cities that are homes for both them and wildlife?

The Romanovs retreat into a bubble of luxury while the czar's forces brutally suppress protests by a starving populace. Rasputin's influence grows, never mind the rumors and scandals. Many people believe Prime minister Peter Stolypin is the last hope of the Romanov Dynasty as reformer-in-chief. He is horrified by the rise of Rasputin. The fight between Stolypin and Rasputin is a battle for influence over Nicholas II. But Stolypin is shot in the Kiev Opera in 1911.

Nintendo was like an impossibly huge wall. But if Mario was the undisputed king, an underdog was about to declare war. For Japanese game maker Sega, mainly known for its arcade cabinets, the battle was about winning the home console market. Sega's Genesis console and its speedy character, Sonic, hit the market.
In the early '80s, sports video games existed, but they didn't really look real. And one visionary entrepreneur had already started tackling the challenge of taking sports from the stadium onto the screen. Electronic Arts kicks off a partnership with football legend John Madden.

Stars helped create us, building and spreading the ingredients for life to develop. But there will definitely be a point in the future when, in the future when, you look up, you will no longer be able to see stars.
For billions of years, stars brought life to the universe. Now, they're dying out in a star apocalypse. What's causing the die-off, and what happens to life when the lights go out?