My first taste of Lake Baikal was in Moscow, 3,300 miles to the west, in the form of drinking water on the No 10 train of the Trans-Siberian Railway. Filtered and purified by voracious sponges that live in its depths, the whole lake is drinkable, making Baikal the source of some 20% of the world's unfrozen fresh water.

After some vodka-laced partying in the Russian capital, the water was more than welcome. After four nights in a second-class compartment on the train, so was the lake.

It was early April and spring was just budding. Snow was melting, birds singing. En route, I saw the great rivers - the Volga, the Ob, the Yenisey - flowing freely without bergs. But the smaller tributaries were rock solid or just beginning to break and Russia was still cold.

Lake Baikal is the most popular stop for Trans-Siberian travellers - it's an opportunity to stretch and exercise limbs, breathe clean air and eat wholesome food. I was staying at Listvyanka, the small resort town on the lake's southwestern shore. From here, Baikal extends to the north like a crooked finger for some 400 miles, covering a surface area of 12,200 square miles - equal to about 2,140 Windermeres. It's the oldest lake in the world, the deepest and the most transparent. Listvyanka has a fantastic little museum, where I learnt many more world record facts and figures, and saw all the weird gastropods (83 of these) and algae (1,085 species) that inhabit the lake as well as the pressure-resistant, see-through golomyanka fish that lives as deep as deep can be.

But you can't really feel statistics. What I saw as we drove past the landmark of Shaman Rock and suddenly came upon the lake was surely one of the planet's most awesome visions. The whole lake was frozen: pure white and shimmering, it looked too pristine to be true, and quite surreal. Sunglasses donned, I could make out walkers, sleds pulled by huskies, a kite-skier, and some kids playing football with a black ball at the edges. There was even a tiny hovercraft taking tourists on excursions. Beyond were the snowy peaks of the Khamar Daban mountains. Gulls dived at the edge of the ice, and two fishermen were sat around a small hole casting lines.

Before such a huge natural wonder, with its blinding light and frozen, impenetrable face, we seek out comfort zones. Listvyanka, which is basically a church, a port and some log cabins, is cute enough and, during the weekends, a market is set up at the edge of the lake providing a social hub, shopping centre and snack bars. Some of the souvenirs are aimed at Russians - dodgy wooden wall-hangings, dead rats (or similar) stretched into furry scarves, grandma-ish necklaces made from the violet charoite stone that is mined here. But there were dozens of stallholders offering fish - pike, sig fish, and the local favourite, salmon-like omul - which they were smoking in birch wood. A bracing wind was blowing off the lake on the day I strolled round the market but for Russians - all of them in serious winter gear and big hats - this was nothing, and family groups stood around tearing off slices of oily fish along with bread, dumplings and bottles of Baltika beer. Suddenly hungry, I decided to do the same, but opted to sit down at the Last Century Café, where the omul came on kebabs, accompanied by borscht and salad.

Food was even better at my accommodation. I had been wary when I was told I was having a "Siberian homestay". It sounded like the kind of euphemistic invitation they may have offered to Solzhenitzyn before shipping him to a gulag. But it's the done thing in Listvyanka, where the hotels are overpriced and still controlled by the supremely inefficient Intourist organisation which dates back to Soviet times.

My host and flatmate was Valery, an aqua biologist, a keen skier, diver and swimmer and, as it turned out, a handy cook. He made wild Siberian mushroom soup, rice with curd, meat and onion dumplings and gathered his own honey and wild berries to make preserves. He made me black tea with this and every other meal, but I bought some (pretty dreadful) sweet Bulgarian wine called Bear Blood for variety.

Morning was omelettes, porridge, fresh wheat biscuits, cooked meats, more tea. This all needed burning off.

So on my second day we walked across the ice to Port Baikal. Although the lake is at roughly the same latitude as London, it had snowed heavily during the night. Siberia is wide open, with lots of steppe land and few high mountains to temper the winds, which bring in extreme weather. In summer there's no relief, as massive electric storms are created when cold air settles above the warm lake and the lake gets as choppy as a sea.

It was hard to imagine waves at all this morning. I was not overly keen on walking on the ice. It was ice-rink slippy, the snow hid any potential cracks, and, I wondered, how safe was it so late in the season? "No problem," Valery assured me. "It's about 1.3m thick. Look, there are cars crossing right now".

A minute later his foot went through right up to the knee. Changing a sock, he said this was "unusual" and that "it must be a hole for fishing". We continued on tiptoe through the new covering of snow. Both of us occasionally slipped on the wind-polished ice and I slipped discreetly into single file behind Valery.

Port Baikal is a pretty little town, with a population of a few hundred souls living in wooden houses. There's no road link to the rest of Russia here and it feels far more remote than Listvyanka. It was election day - the native Buryats, who live and work in an autonomous district, were voting in a referendum on whether to join the Russian political system - and there was piped music and quite a lot of activity. Well, maybe 10 people walking around.

Ringing the lake is dense, dark taiga - the classic Russian forest of silver birch, silver fir, spruce and amber-orange larch - skeletal in spring - and evergreen pines. There are still industrial pressures on the lakeside - pulp and paper plants are the main culprits - but since 1996 Baikal has been a Unesco world heritage site as "the most outstanding example of a freshwater ecosystem" and a string of national parks protect its forests and the elk, bears, ermine and snow rams that live there.

Baikal is a year-round adventure centre. From mid-May onwards, the ice breaks up and there are four months of fluid water for sailing and windsurfing and, in the shallower, warmer coves, swimming. The sky is usually deep blue and the lake gets more hours of sunshine than the Black Sea. If you don't want to walk round the edges, there are horses and bikes for hire. There are also hang-gliding lessons if you want to get an aerial view of Siberia's "Blue Eye". Boats ply up and down the lake visiting islands and remote bays, though be warned: the further north you go the more mosquitoes you will encounter. There are campsites and mountain trails, and deep-lake fishing.

Baikal rewards all your exertions amply. There are private and communal banyas - saunas - all over Listvyanka. I went twice. It's a time to relax, read, detox and warm up. There is a special pleasure to be had looking out of the window at the cold blue from the cosy wooden cabin set at a permanent tropical temperature. Afterwards, there's always black tea - after everything there's black tea - and a chat. Invariably, Russians have interesting stories to tell about perestroika, Baikal's beauty and, of course, Roman Abramovich, who it seems they regard as something of a joker.

Before leaving the region, I visited the town where the train stops - Irkutsk. It's the other reason why everyone breaks their journey at Baikal, for this is the so-called "Paris of Siberia", a nexus of history, culture, architecture and religion which provides the balance to all the natural beauty and wildness of its environs. Like other Siberian settlements, it's a former dumping ground for dissidents, criminals and random non-Russians. It has also long been a trading post for Buryats, Mongolians and other nomadic peoples.

Irkutsk is near to gas and oil wells and is quite rich. People, especially women, dress well here, and the people I met were proudly Siberian. Tamara, my host, was a 69-year-old linguist who had travelled widely in Europe and south-east Asia. Over wine - Bulgarian again, awful again - she filled me in on the local mafia, on life in Vietnam and on the subject of Siberian weather. She missed the cold while living in Hanoi, and she seemed to miss the Soviet Union too. If Valery was a cook, Tamara was a chef - I had a slap-up meal of chicken and mash, knowing I'd be on hydrogenated food once I got back on the train.

It was time to continue my journey. Baikal and the Trans-Siberian go together. They are both big, strange and unique. Much of Russia is an ecological wasteland, and the plains look parched, acidic and doomed, especially after winter when they turn a dirty yellow. But Baikal is a gasp of hope for Eurasia and a blast of healthy living to break up a long sit-down train odyssey. It's some effort to get there, but, on balance - taking into account the smoked fish, the steam baths, the views and the cool beauty of the taiga - it's worth it.

Way to go

Getting there

The Flight Centre (0870 4990042, www.flightcentre.co.uk) offers outward London-Moscow flights, returning from Beijing from £560 inc tax with BA. On The Go Tours' (020-7371 1113, onthegotours.com) 13-day Siberian Highway trip costs £1,229 excluding flights. Stops are Moscow, Yekaterinburg, Listvyanka, Irkutsk, Ulan Ude, Harbin and Beijing.

Further information

Country code: Russia 007, China 0086.

Flight time London-Moscow: 4hrs, 10mins; Beijing-London 13hrs.

Time difference: Moscow +3hrs, Beijing +8hrs.