The Tuareg nomad appeared out of the black night. Wrapping his billowing blue robes tightly around his body, and without saying a word, he squatted on the desert sand next to our fire and warmed his hands against the night chill.
Glancing over at me, he nodded a greeting, unperturbed and seemingly unimpressed by the sight of a small, foreign TV crew in this remote region of northern Mauritania. My jaw, by contrast, was hanging open. For several hours before we paused to pitch camp, we had not seen another living creature, let alone a wandering nomad, as we drove across this vast North African state.
There are strict rules about hospitality in the desert. Food, water and heat must be shared. We cooked bread and meat in the embers of our fire, and the Tuareg ate a full meal with us before he disappeared back into the darkness with just a cheery wave.
It was a magical moment on my journey around the Tropic of Cancer, for the six-part BBC TV series of the same name, and one of the most memorable encounters of my Tropical travels. I have spent the past five years circling the world three times for BBC TV series, travelling first around the Equator, then the Tropic of Capricorn, the southern border of the Tropics. For my third journey I followed the Tropic of Cancer, the line marking the northern border of Earth's tropical region, on my longest and most challenging adventure.
After completing the set, I finally have time to organise my photographs into albums and indulge in some chin-stroking reflection on the trips and the travel value of following the three lines.
There are few reasons why I would normally be meeting a mysterious Tuareg traveller in the middle of nowhere, but following those imaginary lines around the planet forced me to travel off the beaten track and led to many strange and wonderful encounters. Tracking the lines would take me to a city or a tourist sight one day and then a remote, forgotten region of the world the next.
The idea behind the journeys was never to follow the lines too obsessively. Instead I zigzagged along them through the Tropics, and used the lines as a way of discovering more about the region blessed with the greatest natural biodiversity on Earth, and burdened by the greatest concentration of human suffering. The Tropics are the most fascinating, extraordinary and troubled region of the world.
Each of my journeys blended travel with current affairs, because learning more about the places we visit makes for a more interesting experience, a more rounded adventure. So, while I had three amazing adventures, I also learnt more about life in the Tropics, the issues affecting the developing world and about countries we rarely see on the television. Following the three lines took me through 36 countries, ranging from Gabon to Colombia, Indonesia to Algeria, Oman to Paraguay. And not just into their capitals for a brief stopover but often into remote regions of the country.
The result, at least for me, was journeys that were exhilarating, fascinating, shocking, upsetting, frightening and magnificent. There was a not a single day that passed on any of the trips when I wasn't learning something new or being confronted by an extraordinary new sight.
The mechanics of these trips, and the sheer logistical challenge of following the lines, was unlike anything I had attempted before. We travelled overland by four-wheel drive, boats, canoes, taxis, train, vans and rickshaws. But roads, railway lines and airlines did not make it easy by neatly following the lines of latitude. Although we planned our route imaginatively, we still had to make regular detours away from the line.
Yet detours have their own charm. Stopped from travelling east along the Tropic of Cancer from Western Sahara into Mauritania by one of the largest minefields on the planet, we had to divert far to the south, and crossed the minefield on badly marked tracks with a driver who didn't know the way. What started as a diversion was transformed into one of my favourite journeys as we travelled a route from Mauritania into Algeria that foreigners hadn't taken for decades, and fed a roaming Tuareg nomad along the way.
It was these encounters, some planned and many spontaneous, that really moved and moulded me on my journeys.
To put my daily worries into immediate perspective, I always remember the wizened old man I met who was earning a pittance for carrying a heavy bunch of firewood in the Democratic Republic of the Congo, the country cursed by one of the most violent conflicts on the planet since the Second World War. As travellers, we all live for memorable encounters such as these, however brief. Those glorious moments of warm, human interaction, across cultures and races, can make an adventure, or even just a weekend holiday, truly unforgettable.
The interaction doesn't necessarily have to be with humans. Travelling around the Tropics, I was mugged by a mischievous orangutan called Pan, who lunged to grab my bag of supplies in Borneo's Tanjung Puting national park, a rare wildlife sanctuary, open to tourists, on an island scorched and devastated by logging companies and palm oil plantations.
Orangutan numbers have collapsed by two-thirds since 1990, and conservationists in the park run an orphanage for baby orangutans. Without their parents, orangutans can find the rainforest intimidating, and they need to be taught how to forage in preparation for their eventual release into the wild. As I carried a hefty young infant called Osbourne into the rainforest on a training trip, he grabbed me nervously around the neck, glancing around at the unfamiliar surroundings.
I introduced him to a likely tree, and lifted him on to the lower branches. But the poor thing didn't want to go, and climbed back into my arms. I tried again, lifting him on to a branch, patting the tree, and making encouraging noises, as if I was taking the stabilisers off his bicycle. Osbourne looked up at the tree, looked back at me for reassurance, and began to climb. It was food for the soul.
The landscape of the Tropics also had an extraordinary impact on me. From the giant walls of orangey-red Uluru in the middle of Australia, to the wild, jagged landscape of southern Algeria, the Tropics have endless variety and grand scale. Heading east through Brazil, following the Equator on small boats along the Uapes River, I travelled through remote, spectacular areas of the Amazon rainforest, where verdant flora clung to every patch of land, right down to the waterline.
One of the most exhilarating moments of pure travel joy came on the Tropic of Cancer as we drove through Libya, a former pariah state and now a holiday destination that offers the best introduction to the vast Sahara. Driving over giant sand dunes and through a rolling sea of sand in the south of the country, we arrived at the exotic Ubari lakes, a string of gorgeous desert oases formed from vast pools of water that lurk deep under the dry desert. Fringed by palm trees and rushes, the lakes are a breathtaking, picture-postcard image of desert paradise, and taking a dip in the salty water offered a chance to marvel at the most beautiful desert oases on the planet.
But if landscapes were a competition, Madagascar would be the winner. Crossing the island, the fourth largest in the world, the black road carved across a sandy plain, through a low forest of thorns, a plain of octopus trees, then a plain of baobab trees, some with high, carved caves where herdsmen can shelter. Again and again, I was left gasping at the beauty in front of me. Looking back, I can see I took a few risks. I travelled through minefields, attempted to surf the infamous Pororoca in Brazil, a tidal bore wall of water churning with lumps of wood and sharks, and wrestled with a masked female Lucha Libre wrestler in Mexico.
But I always tried to use every encounter and adventure as a means of learning more about the world, and at least they were experiences that will linger in my memory.
As will the culinary offerings of the Tropics, including Zebu penis soup in Madagascar, and a breakfast menu of grilled squirrel and fried caterpillars in Laos.
If I had to pick just one experience on my Tropic journeys that best represented the joys and difficulties of following those three circular lines, it would be travelling into Burma.
The Tropic of Cancer runs across Burma, and although the BBC and other film crews are banned from entering by the military regime, the team and I were determined to follow the line into a country with one of the most repressive governments in the world.
So, two colleagues and I used a ramshackle smugglers zip-line to cross the river border from India into Burma. Guided by an extraordinary young Burmese woman from the Chin ethnic group, we headed on foot for a village in the Chin ethnic area of the country, trekking through sweltering jungle. The Chin were the reason for our covert trip. A Christian ethnic group numbering around 1.5 million inside Burma, they live directly on the Tropic of Cancer, in an area called Chin State.
The team and I had been told the Chin endure appalling human rights abuses under the Burmese military regime, and we wanted to meet them and learn more about their difficult lives.
But travelling undercover into Burma was risky. Our journey took us into a region with more than 50 Burmese military bases. We trained and prepared for the journey, and were carrying food, water, ropes, machetes, locator beacons, camouflage equipment and survival kits in case we had to hide in the jungle or evade pursuing soldiers.
We travelled quickly and quietly on a long, sweaty march through the jungle to the Chin village. Stumbling into the village was an enormous relief, and huts emptied as locals came out to welcome us, the first foreigners most of them had ever seen.
The Chin were desperate to tell the outside world about their treatment at the hands of the Burmese army. The villagers explained that forced labour, torture, rape, arbitrary arrest and summary execution by soldiers are commonplace in Chin State, part of a Burmese government policy to suppress the Chin people.
It was a tense visit, and we never felt safe. Late at night a breathless runner arrived to warn us a patrol of soldiers had arrived in the next village and was likely to be heading in our direction. We considered hiding in the jungle, but the danger was just too great. We grabbed our bags and fled into the night, back towards the safety of India. The experience gave me a brief sense of the sheer terror that villagers and the Chin endure on a daily basis under their military rulers.
The Chin are a forgotten people, in a forgotten region of the world. Few foreigners have travelled into their area of the planet, let alone film crews. But following the Tropic of Cancer forced us off the beaten track and took us to hear their stories. That single adventure encapsulated everything that was thrilling, shocking, and memorable about following the imaginary lines.
It's an experience that I'm keen to share. And it's a style of travelling that I would recommend. If you're looking for a holiday that offers a real sense of purpose, then follow a line. Find a clear start point, and identify a clear destination. It doesn't have to be a journey around the Tropics, around the planet, or across a continent. It could be an adventure along a river, or a train journey.
Just don't pick a holiday where you sit by the pool wearing an iPod. Because your memories of the trip won't even last as long as your suntan. And where on Earth is the fun in that?
Simon Reeve travelled through 18 countries on his latest journey around the Tropic of Cancer. The BBC DVD boxset of the Tropic of Cancer series is available now, priced £24.99. For more information, visit simonreeve.co.uk
Real lives: Conflict and communities
The intense privilege of being on the journey was brought home to me early. Following the equator across Africa, I went to a refugee camp on the Kenyan–Somali border. Inside the camp I met Fatimah, 23, who had been there for most of her life. She was literate, fluent in English and bursting with capability and promise. But Fatimah was forbidden from travelling more than a few kilometres from the camp by the Kenyan government. She was unable to go home because of conflict and chronic instability in her home country. Yet an accident of birth and my British passport was allowing me to travel the world. Her story still haunts me.
I also met Margarita Mbywangi, a chief of the Aché people, in a remote village while following the Tropic of Capricorn through Paraguay. Margarita's story was as extraordinary as the tale of her people. When she was five years old her village was attacked by farmers. She was sold into servitude before she found herself with a family who gave her an education, and eventually she returned to her community to give them wisdom learnt from the outside world. She is an extraordinary woman, and someone it was a privilege to meet.