Tibetan toilets are legendary among travellers, and as we found out, they deserve to be. They reach levels of filth, stinkiness, discomfort and/or simple bad plumbing that are seen in few other places in the world. During long days of driving in Land Cruisers over dusty, bumpy roads, the members of the tour group started to bond by swapping the inevitable toilet stories. Jeanette had managed to drop her toilet roll into the hole while in mid-squat, and to prevent it blocking up the primitive plumbing system had selflessly stuck her hand down into the mire to retrieve it; Gwen's problem was with a powerful but mis-directed flushing mechanism, that propelled the toilet's contents up out of the pan, under the door, and across the bathroom floor; Bob, who stands well over six feet tall, complained that to use a squat toilet he had to assume a posture similar to that of a giraffe bending down to drink. Most of our toilet stops, however, were out in the open, in the bracing air of the mountains. High altitude causes the body to become dehydrated, and we were all drinking huge quantities of water - three litres a day or more - to try and stave off the effects of altitude sickness. Barely an hour went by without one or more of the Land Cruisers making a comfort stop; the boys would line up with their backs to the road, the girls would try to find a secluded spot behind a gravel mound or small hill. Usually it turned out to be less secluded than they thought, to the entertainment of the passing Chinese truck drivers.

On the first day out of Lhasa, we travelled south along the Friendship Highway, then turned east to follow a road along the south bank of the Yarlung Tsangpo river. Leaving the Land Cruisers at a small guesthouse near the riverbank, we boarded an open barge which ferried us between the sandbars and across to Samye monastery - the oldest in Tibet, dating back over 1,200 years. Some sections were destroyed during the Cultural Revolution and are now being rebuilt; the ugly dark-green chörten (stupa) near to the main temple looked more like Soviet modernist architecture than anything to do with Buddhism. The monastery guesthouse was simple but fairly comfortable, although the toilets were "basic" and the girls' room turned out to be home to a mouse, who hopped from bed to bed and hence acquired the name of Britney.

After a morning looking around the monastery and the nearby village, we decided that we'd all like to visit the Chim-puk hermitage, a meditation retreat in the hills that is home to a community of monks and nuns. At the time, I didn't realise what sort of journey was involved to get there. We all piled into the back of a truck which set off on a track across a flat plain, which soon turned into a winding dirt road up the mountain. The road was narrow, the truck was wide, and it lurched sickeningly from side to side. I wasn't the only one who was making plans about which side to jump out of when (not if) it went over, but somehow we stayed upright all the way to the parking area at the end of the track. No one complained about making the remainder of the climb on foot.

A little way up the hill from the main village, Bob, Gwen and I stumbled on a collection of tiny houses arranged round a central courtyard, where a tree was bursting into pink flowers. These quarters were home to three nuns, who invited us in and showed us around. Behind the main entrance, all of the rooms were carved out of the rock - the kitchen had a neat collection of pots and pans, and a shelf with a scarcely adequate store of instant noodles and rice; the meditation room was a dark alcove with a few Buddha images; in the bedroom, a dirty mirror was ordained with photos and paintings of important lamas and Buddhist deities. Everything was tiny and cramped, the contemplative lives they led were obviously hard and isolated - but the nuns were incredibly hospitable to a bunch of foreigners who'd decided to snoop around their village and point cameras everywhere. I guess it's a cultural exchange of sorts: they gave us a brief but very special insight into a completely different lifestyle, we gave them a good laugh with our digital cameras. I'd like to think that the overall good karma that was generated was one reason we all managed to get down the mountain again in that scary truck, and arrived back at Samye in one piece.

Back in the Land Cruisers the next morning. West, then southwest, over the Kamba-la pass (4,794m) from where we looked down on the deep blue waters of the Yamdrok-tso lake. On past Nangartse, climbing again into the mountains, through rain and hail, breaking down in the middle of a snowstorm, watching the driver remove a bit of the engine, bash it on a rock, then put it back. Overnight in Gyantse and a morning tour of the Kumbum, an impressive multi-storey structure packed with murals and sculptures. West again along the southern Friendship Highway, missing our lunch stop because of some unspecified problems with the PSB (Public Security Bureau), and on to Shigatse. Tashilhunpo monastery in Shigatse is the seat of the Panchen Lamas, who traditionally hold a position of authority comparable to that of the Dalai Lamas. Now that pictures of the current Dalai Lama are illegal, images of the 10th Panchen Lama (who died in 1989) are common in temples and Tibetan homes. He seems to be acceptable to the Chinese authorities, while also being held in very high regard by Tibetans. Although he grew up in the control of the Chinese, and was at first seen by many as being generally supportive of the Chinese occupation, the situation changed when in 1961 he sent a strongly worded letter to Mao Zedong documenting the atrocities and cultural destruction that the Chinese forces had brought to the region. For this he was imprisoned and tortured in China, and remained there after his release in 1978, only returning to Tibet at the end of his life.

The journey continued west from Shigatse, over the Tropu-la pass (4,950m) to Sakya monastery, then on to Lhaste where we stayed overnight in a filthy guesthouse with a serious rodent problem. From Lhatse the road turned southwest, and the snowy mountains that we'd seen in the distance disappeared from view, replaced by brown-red sedimentary rock formations, their contortions reminding us of how they were churned up from the bottom of the sea when the Tibetan plateau was formed. Shortly after the Gyatso-la pass (5,220m), I saw the Land Cruiser at the front of our convoy pull over to the side of the road, and the passengers jump out excitedly. Soon, I was doing the same - as we came round the corner, a vast section of the Himalayan range was revealed before us. The famous 8,000-metre-plus peaks: Makalu, Lhotse, Everest, Cho Oyu. We could see a thin plume of water vapour streaming away from the summit of Everest. I felt exhilarated that I was here and seeing this, a view that I'd seen only on television or on postcards, and which I had somehow doubted actually existed. Everyone in the group was elated that we had a clear sky that morning, since the previous few days had been overcast, and as we drove on, descending from the top of the pass, we couldn't take our eyes off the mountain. By the afternoon the plume from the summit had thickened, and the top of the mountain was hidden by cloud as we passed Ronghpu monastery where we had planned to spend the night. But after reports from the drivers that the rooms there were even filthier than the ones at Lhatse we drove on, to Everest Base Camp.

Just the idea of sleeping in the shadow of Everest was exciting. Base Camp was a much smaller affair than I'd imagined, probably because most expeditions choose to climb from the Nepali side - it's easier, both technically and administratively. (However, one big advantage of the Tibetan base camp is that you can drive right into it, instead of having to hike for weeks to get there.) May is peak season for expeditions, since the best weather for the climb comes in late May and June, just before the monsoon. There were many expedition tents dotted around the site, and a few more "permanent" tents acting as teahouses and restaurants. We stayed in the "Hotel California" tent, which was a food tent by day, with benches that doubled as beds at night. Sleep was difficult, at first because of the flapping of the tent in the strong wind, but later because of the thin air. I kept waking up gasping for air, sucking in huge breaths and filling my lungs right to the bottom. It felt like breathing was having no effect. At 5,200 metres, the height of Base Camp, the oxygen content of the air is about fifty percent of that at sea level.

The camp toilets were situated up a slight incline from our tent, and even this minor physical exertion first thing in the morning left me panting. Conditions were primitive - just a hole in the floor, through which you could see mini-mount Everests with their summits growing dangerously close to floor level. These stinking peaks deflected the icy winds up through the hole, threatening frostbite to any exposed flesh. It was not a place to linger. But the morning's views made up for everything - the sky was clear again, and at about 7.30am I climbed the small hill just outside the main group of tents, and stood by the garlands of prayer flags arranged on the top. Gradually, the eastern faces of the mountain were illuminated by the rising sun, and turned a pale golden colour. The wind was strong, and the prayer flags flapped violently, but I stood there for a long time, taking far too many photos, until I lost feeling in my feet and was forced back to the relative warmth of the tent to defrost them. A couple of hours later the summit started to plume, and I tried to imagine what sort of winds were raging up there. It seemed impossible to believe that anyone could climb that thing. Later in the day I found another small hill, on top of which was a memorial to Mallory and Irvine, who started their 1924 attempt on the summit from the Tibetan side, and who were last seen somewhere above 8,000m.

We were so lucky with the weather. The two days we spent in the Everest region were mostly clear, but once we left Base Camp it became cloudy again. We stopped overnight at Tingri, a dusty dust-bowl of a town where dust-storms raged through the afternoon and evening. The dust covered all our bags, coated our skin and hair, irritated our noses. Next morning we crossed the last high pass, Tong-la (5,120m) and continued south to Nyalam, right on the edge of the Tibetan Plateau. Past Nyalam, it felt as if we were dropping off the edge of the world as the road wound steeply down the side of a gorge. Trees appeared. Waterfalls. Flowers. After weeks of driving across a desert, the transformation seemed miraculous. There was oxygen in the air, I felt suddenly energised as my higher brain functions started up again and I lost the leaden feeling that had affected me at Base Camp. That night at Zhangmu we had our first showers for days, and the reddish-brown colour of my skin, which I thought might be a good sun tan, washed off and disappeared down the plughole.

Tibet is a place of intensity - the scorching sun, freezing winds, thin air, and the extreme religious devotion of the people. It left impressions on me unlike any other country I've been to. My feelings while travelling through it seemed intensified by a core of sadness that I sensed at its heart, and it was impossible to forget that this culture is under grave threat. It's easy to think that a society that survived the Cultural Revolution can survive anything, but the new assault on the country is different, more systematic, more organised, less histrionic but far more focused than the madness of the 1960s. The grand civil engineering schemes of which the Chinese are so proud are primarily designed to encourage increased Han Chinese immigration into the Tibet Autonomous Region - already Chinese are believed to outnumber ethnic Tibetans in Lhasa, and probably also in many other parts of the region. For obvious reasons, accurate figures are difficult to obtain. The 1980s saw some improvements in religious and cultural freedoms, but recently there has been a new crackdown, as the Chinese government becomes increasingly paranoid that any relaxation of its grip could send the Motherland in the same direction as the former Soviet Union. The future is uncertain, but the exiled Tibetan community scattered around the world provides at least some hope that even if the Chinese continue to overrun the Tibet Autonomous Region, the traditions and culture of the Tibetan people will not be lost to the world.