Let's not go through that again
The Darwin Awards are bestowed annually upon humans who are so mind-bogglingly dim that they earn themselves deselection from the gene pool for dying in the most moronic of circumstances.
My expedition partner, Leon McCarron, and I might well have qualified for it following our - on hindsight, silly - decision to walk through the 18-kilometre Zhongnanshan Tunnel, the world's second-longest road tunnel which cuts through the middle of the Qinling Mountains in the southern part of China's Shaanxi province.
Feeling rather like when the Fellowship of the Ring decided to brave the mines of Moria, we decided to try our luck at walking through it, because we were behind schedule and faced even further delays if we walked up and down all the passes and descents along the small roads that wound through the mountain range.
We had, after all, already crossed the Gobi Desert and walked down the Yellow River valley, covering more than half of our 5,000-kilometre Walking Home From Mongolia expedition so far. At this point, we'd take any short cuts we could get.
We are heading for the Yangtze, about three more weeks' walk away. In the past week, we walked south out of Xian's city walls,and then through the city's giant grid system of buildings and roadworks before finally reaching the countryside.
At the edge of Xian, we reached a steep hillside, beyond which a patchwork of fields spread as far as the eye could see - which on that particular day was probably only about one kilometre, as the air was filled with a damp, grey smog. Three hours later, the flat monotony ended, and towering before us were the giant Qinling Mountains - huge limestone cliffs jumping out from the ground like a battalion of soldiers determined to stop anyone passing.
In fact, through history, these mountains have proved a brilliant defensive barrier against invading marauders, as there are only a handful of crossing points through the cliffs. They also act as a rain barrier that contributes to the aridity of the north, and in their deciduous forests is an incredible ecological diversity, which includes even wild pandas and leopards. It is probably partly because of the different climates on the two sides of them that the mountains have been considered a symbolic barrier between the elusive concept of 'northern' and 'southern' China.
Although never officially defined, in general, the north has been seen as wheat-growing, Putonghua- speaking and horseback fighting, while the south grows rice, speaks mainly non-Putonghua dialects, and is better at naval warfare.
The night before we reached the Zhongnanshan Tunnel, we passed a village where a group of migrant workers, gathered around a little fire, invited us to sleep on a couple of old doors (insulation from the cold ground) in their house. The next day at 4.30am we climbed up a verge onto the stilted motorway, sneaked past a snoozing policeman in a guard hut, and entered the tunnel.
Neon lights stretched before us into the smog, and in the distance was the thundering sound of a still-unseen vehicle heading towards us from the middle of the mountain. The air was warm in here, and there was an alarming sense of thick fumes in the air.
We should have more seriously considered the risk of suffocation, but had rather assumed that as this is quite a new tunnel (opened in 2007) it would have state-of-the-art ventilation systems. But as we walked fast into it, our adrenaline speeding our steps, we noticed that only about half of the fans on the tunnel ceiling were working.
We gasped and pressed forward, and every 250 metres there was a little escape room, where we sometimes stopped to try and find fresh air. After an hour, Leon had a headache and I was feeling dizzy, but we were not sure whether it was due to the fumes, our early start, or just adrenaline and nerves.
Occasionally, cars would pass by, with drivers staring unbelievingly at two haggard-looking Caucasians walking through the tunnel so early in the morning. By the end of the second hour, we were both desperate to reach the end and so, for the first time in the expedition, started to jog, rather than walk. It was now far too late to turn back - we were past halfway, but we really wondered whether we would make it.
(Well, obviously we survived, but to find out what actually happened, you will have to wait for the television show.)
Back in fresh air, we continued along the smaller mountain roads again and stumbled upon the Zhashui Caves, crossed the valley of the Han River, and arrived in the city of Ankang. We are now truly in southern China, and before us, about 10 days' walk - and many mountains away - is the Yangtze.
Rob Lilwall's previous expedition, Cycling Home From Siberia, became the subject of an acclaimed motivational talk, a book, and a National Geographic television series. Every week in Health Post, he will write about the progress of his latest expedition, Walking Home From Mongolia, which is in support of the children's charity Viva. www.walkinghomefrommongolia.com