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Simon should have started this final leg of his journey by following the Tropic of Cancer through the far south of China, but Beijing officials declined to provide the team with visas, a reminder that the Chinese government still has a difficult relationship with foreign media.

So, instead, Simon began this final leg of in Laos, just to the south of China, at the very centre of the infamous Golden Triangle, the tri-border river junction between Burma, Thailand and Laos. The Golden Triangle is known globally as a major centre of heroin production, but that hasn't deterred a group of Chinese businessmen from starting construction of a vast "Golden Triangle Entertainment City" on the remote banks of the Mekong River, with hotel resorts and plush casinos, where Simon tries his hand at cards.

Heading across Laos, inside the Tropics and parallel with the Tropic of Cancer, Simon takes a boat down the Mekong river, eats grilled squirrel and caterpillars in the beautiful city of Luang Prabang, and learns that Laos became the most heavily-bombed country in history during the Vietnam War.

He meets bomb disposal experts struggling to clear millions of tons of unexploded bombs that litter the country, and discovers a live cluster bomblet (which has a killing radius of 30 metres) lying in the middle of an unmade road.

After several days of mountain driving, Simon arrives in Vietnam, where he visits a sanctuary for bears rescued from bear farms, and investigates the trafficking of young women into China. Victims of the gender imbalance between men and women in China, they're sold to Chinese husbands who can't find local brides. 

Simon then heads to Taiwan, one of the few economic success stories in the Tropics, where he travels by bullet train and visits the typhoon-damaged central highlands.

The incredible journey ends in the spectacular tropical islands of Hawaii – a paradise on earth for most people. But Hawaii has some of the dirtiest beaches in the world and is also the extinction capital of the planet, proof that even these remote islands aren't immune from the global problems afflicting the Tropics.

As he finishes his trilogy of journeys, the issues affecting Hawaii offer Simon a chance to reflect on how the relationship between mankind and the environment has become perhaps the most important issue in the Tropics, and the entire world.