The Building of the Earth begins in the world's deepest valley: that of the Kali Gandaki river in the Himalayas. Its temperatures range from those of the tropics in its lower reaches to that of the poles higher up. It shows how creatures become adapted to living in certain environments.

The higher that Attenborough travels, the more bleak and mountainous is the terrain, and the more suited to it are the animals that live there. However, such adaptations are comparatively recent: these mountains were formed from the sea bed some 65 million years ago.

To show the force of nature responsible for this, Attenborough stands in front of an erupting volcano in Iceland and handles a piece of basalt; the Giant's Causeway is an example of what happens to it over a great length of time. The Icelandic volcanoes represent the northern end of a fissure that is mostly underwater and runs down one side of the globe, forming volcanic islands en route where it is above sea level.

It is such activity, known as plate tectonics, from deep within the Earth that pulled apart Africa and South America and created the Atlantic Ocean. Footage of the eruption of Mount St. Helens in 1980 shows what decimation it caused. However, this pales in comparison to the destruction caused by Krakatoa in 1883, which Attenborough relates in detail. When such pressure beneath the Earth shifts, it results in hot springs and caverns — which themselves support life.