Hours before Mehmed II launches his final assault, an ominous sight shakes both sides. Rumors of a 40-ship fleet's imminent arrival have swirled for weeks, but it's exact whereabouts remained unknown. Ottoman cannons reduced the city walls to rubble, and Venetian reinforcements arrived too late. The conquest of Constantinople ushers in a new era for the Ottoman Empire, being a dominant force in world politics for 300 years. Mehmed II, in many senses, changed the nature of world history.

Immense shoals of fish throng our shallow seas. Small fish, in turn, sustain bigger ones. The rich coastal seas are the fishing grounds of our planet and can provide an abundance of food for wildlife and humanity. The seas fringing land make up less than a tenth of the world's oceans but 90 percent of marine creatures live in coastal waters, from fearsome sharks to lowly urchins. Protecting these habitats is a battle humanity must win.

A rain forest is the richest habitat on Earth. Exactly how many species rain forests contain is unknown, but it runs into millions. And new ones are discovered every week. There are some, like the clouded leopard, we still know virtually nothing about. Although they cover just seven percent of the world's land area, jungles play a vital role in the health of the planet .
Jungles and rainforests are home to an incredible variety of species like preening birds, intelligent orangutans and remarkably ambitious ants.

The search for climate change solutions requires passion, resources and a sense of urgency -- three qualities Bill Gates clearly possesses. Bill has founded a start-up called TerraPower. After extensive computer modelling, the idea showed promise. Its new reactor greatly reduced the chance of human error. Fuelled by depleted uranium, the travelling wave reactor functions like a slow-burning candle and requires refuelling only once every decade. Bill and his team believed they had finally developed the ideal energy source, a reactor that was clean, efficient, and most importantly, safe.

The polar regions of our planet may seem beyond the reach of most of us, but they are not beyond our influence. We, unintentionally, are changing these frozen worlds, and these changes will not just affect the poles but the whole planet. On the unforgiving frontier of climate change, polar bears, walruses, seals and penguins find their icy Edens in peril.

A fifth of the land on our planet is covered by desert. Deserts may appear to be barren and empty, but they are of crucial importance to life. For those that can overcome their challenges they provide a vital refuge. The Grasslands are one of our planet's most productive landscapes. They support the greatest aggregations of large animals on Earth. Cameras follow desert elephants seeking sustenance, bison roaming North American grasslands and caterpillars living the good life underground.