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.
In the third programme presenter Simon Reeve follows the Tropic of Capricorn as it cuts through three regions of Australia (Western Australia, the Northern Territory, and Queensland) and the vast wilderness of the Outback.
Simon saw the fin of a humpback whale diving in the Exmouth Gulf
This is not the Australia of Neighbours and Home and Away.
It is the heart of Australia, a remote and spectacular place populated by extraordinary people and wildlife - and the scene of some intractable and unexpected social problems.
The journey begins as the line hits Western Australia near Ningaloo reef, on the migration route of the mighty humpback whale.
Simon witnesses the unforgettable sight of female whales nursing their calves before the long trip south to Antarctica, but their pristine sanctuary may be under threat from plans to build a vast salt pan nearby.
Western Australia is rich in natural resources, but this has not always been a blessing.
Heading east to beautiful Karijini National Park, nearby Wittenoom Gorge is a region with a sinister history and was the location of Australia's biggest asbestos mine.
Thousands have died after working at the mine, and the authorities, believing it is the worst asbestos-contaminated site in the world, are demolishing the nearby town.
But a few stalwarts have refused to leave, including a tough Outback couple determined to stay in the ghost town.
Next, Simon hops aboard a three-car road-train which powers its way across the Outback to Newman, the site of the world's largest open-cut mine, which sends millions of tons of ore to fuel China's booming economy.
Newman is attracting workers from across Australia, drawn by huge salaries.
This is the last town before the unpopulated deserts of central Australia.
Simon's next stop is Alice Springs, a remote town in the Northern Territory, but also the world centre of Aboriginal art.
Many Aboriginal communities in the Northern Territory are in a desperate state, in societies troubled by violence, child abuse and poverty.
Simon arrives at a crucial moment: a powerful government taskforce has been established to retake control of communities. In the shadow Uluru (Ayers Rock) an Aboriginal community lives in Third World conditions.
In Queensland, Simon arrives in cattle-country and stays with a couple whose 183,000 acres have not seen significant rain for seven years - they live in a dustbowl of biblical proportions.
Heron Island is home to an eminent marine research centre
Some experts say this is the first time climate change has had a serious impact on a developed country.
He heads east along Capricorn to stunning Heron Island, home to one of the world's pre-eminent marine research centres.
The island is at the southern end of the Great Barrier Reef, where climate change threatens to wipe out the coral and the rich wildlife which thrives on it.
Series Producer: Sam Bagnall
Presenter: Simon Reeve
Director: Louise Turner