Bbc planet earth 02 — mountains

Human beings venture into the highest parts of our planet at their peril. Some might think that by climbing a great mountain they have somehow conquered it, but we can only be visitors here. This is a frozen alien world.

This is the other extreme — one of the lowest hottest places on Earth. It's over a hundred meters below the level of the sea. But here a mountain is in gestation. Pools of sulphuric acid are indications that deep underground there are titanic stirrings. This is the Danakil Depression in Ethiopia, lying within a colossal rent of the earth's surface where giant land masses are pulling away from one another. Lava rises to the surface through this crack in the crust creating a chain of young volcanoes.

This one, Erta Ale, is today the longest continually erupting volcano on the planet, a lake of lava that has been molten for over a hundred years.

These same volcanic forces also created Ethiopia's highlands. 70 million years ago this land was just as flat and as deep as the Danakil Depression. Molten lava rising from the earth's core forced up a huge dome of rock 500 miles wide, the roof of Africa. Over millennia, rain and ice carved the rock into a landscape of spires and canyons. These summits, nearly 3 miles up, are home to some very remarkable mountaineers — Gelada baboons. They are unique to the highlands of Ethiopia.

The cliffs where they sleep are for expert climbers only, and Gelado certainly have the right equipment. The strongest fingers of any primate and an utterly fearless disposition. But you need more than a head for heights to survive up here. A day in a Gelado's life reveals how they've risen to the challenge.

For all monkeys morning is grooming time, a chance to catch up with friends. But, unlike other monkeys, Gelados chatter constantly while they do it. It's a great way to network while your hands are busy. But these socials can't go on for too long. Gelados have a busy daily schedule and there's work to be done. Most monkeys couldn't live up here. There's no food and few insects to feed on. But Gelados are unique — they're the only monkeys in the world that live almost entirely on grass.

They live in the largest assemblies formed by any monkeys. Some groups are 800 strong and they crop the high meadows like herds of wildebeest.

The Gelados graze alongside Walia ibex, which are also unique to these highlands. These rare creatures are usually very shy but they drop their guard when the Gelados are around. You might expect that grazers would avoid each other's patch but this is a special alliance from which both partners benefit. It's not so risky to put your head down if others are on the lookout. Ethiopian wolves — they won't attempt an attack in broad daylight. But at dusk the plateau becomes a more dangerous place. With the grazing largely over there's a last chance to socialize before returning to the sleeping cliffs.

An early warning system puts everyone on the alert.

Their day ends as it began, safe on the steep cliffs. The Ethiopian volcanoes are dormant, but elsewhere others still rage.

Volcanoes form the backbone of the longest mountain chain on our planet — the Andes of South America. This vast range stretches 5,000 miles from the Equator down to the Antarctic. It formed as the floor of the Pacific Ocean slid beneath the South American continent, buckling its edge.

At the southern end stand the mountains of Patagonia.