The Tropic of Capricorn runs around the southern hemisphere for about 22,000 miles, crossing some of the most remote and unexplored regions of the world.
The Atacama desert receives just 2% of the moisture in the Sahara
The final leg of the journey sees Simon Reeve follow Capricorn across South America from Chile, over the Andes to Argentina, though land locked Paraguay to the Atlantci coast of Brazil.
South America offers up some of the most spectacular scenery of the whole journey, from the high Andes to the awesome Iguacu waterfalls on the Brazil and Argentina border.
It includes the largest and most vibrant city in the southern hemisphere in Sao Paolo and an extraordinary range of people and places.
The line hits land on the Pacific coast of Northern Chile, near the city of Antofagasta.
From here Simon hitches a lift on a gigantic train through the Atacama desert, thought to be the driest place on earth.
The train is servicing one of Chile's largest copper mines - sales of the metal have turned the country into one of the most sucessful Latin American economies.
But copper mining needs water - a scarce resource - and Simon visits the indigenous people of the area, the Atacamena.
They are fighting plans from the mining companies to extract water from the mountain lakes that form a crucial part of the local ecology.
From here, the Tropic crosses into Northern Argentina over a pass 4,000 metres up in the Andes.
On the other side of the border Simon's next stop is a ranch, where he joins the roundup of a flock of Vicuna, the smaller and rarer cousin of the Llama - once hunted near to extiction for its fabulously valuable wool.
This part of Argentina is also home to the Wichi people, who live a sustainable life in the forests, where Simon risks the wrath of the local bees in a hunt for honey.
This way of life though is threatened by the giant bulldozers clearing the forest for agriculture.
Neighbouring Paraguay has lost 90% of its forest, mostly for the intensive cultivation of Soy, destined for European and Chinese animal feed.
It accounts for almost a half of the country's exports, but has come at a huge cost and farmers and smallholders have been driven from the land.
The country has a dark past of dictatorship and human rights abuses, and is still gripped by corruption and a thriving black market.
On the Paraguay's eastern border lies Brazil. And after flying over the vast Iguacu waterfalls, Simon heads for Sao Paolo a vast metropolis of 20 million people.
It is the centre of Brazil's wealth creation, with more helicopters buzzing around than any other city apart from New York.
And in the crime ridden Favelas there is a project helping to give help to young people and lift them out of poverty.
After months of travel, the journey around the world ends on a beach in the small town of Ubatuba.
Series Producer: Sam Bagnall
Presenter: Simon Reeve
Director: Dominic Ozanne