Lake Khövsgöl holds close to seventy per cent of Mongolia's fresh water, 136 kilometres of it in the far north near the Siberian border, and the country calls it the Mother Sea rather than a lake. Only small mountain streams feed it, and they bring almost no nutrients, so there is nothing to cloud the water. A white disc lowered from a boat stays visible more than eighteen metres down. That clarity comes with age. Most lakes silt up and disappear within twenty thousand years. This basin is a rift more than two million years old, among the oldest freshwater lakes left anywhere. Along the shore stand ovoos, cairns of stone hung with blue silk scarves, and passing herders add a rock and walk round three times clockwise.
No road goes round the lake, so the shore is ridden rather than driven. On a hardy Mongolian horse you weave between larch trunks, the water flashing on one side and the forest climbing to the peaks on the other. That is how you reach coves nothing else touches. An open canoe pushed out from a quiet cove goes over shallows so clear that pale stones and sunken logs seem to hang just under the hull. The water is cold and clean enough to drink off your fingers. For a few weeks in summer the lakeside meadows fill with wildflowers while Darkhad and Tsaatan families raise their gers and graze horses and reindeer. North of the lake, days away on horseback, perhaps two or three hundred Tsaatan are left in some thirty families, living in bark-and-canvas tipis rather than felt gers and moving their herds through the taiga. Winter turns the whole thing to clear ice over a metre thick, and in early March the Ice Festival brings sledges, skaters and wrestling out onto it. Soviet convoys used to drive fuel tankers straight across from Siberia, until thirty or forty of them went through and the leaking oil began to threaten water you can drink. The crossing was outlawed in the 1980s.