Lake Baikal is the deepest and oldest lake on Earth, a crescent in the mountains of southern Siberia holding about a fifth of the planet's unfrozen fresh water. It does that by being deep rather than wide. It is not a basin but a rift, a place where two slabs of the Earth's crust are pulling apart, and the floor lies 1,642 metres down. The gap is still opening, about two centimetres a year, and has been filling for 25 million years. The water is clear enough to see forty metres down, so from a boat you can follow the bottom down until it goes black. In summer boats work the wilder shores, cliffs going straight into the water, hidden coves, rock pillars standing offshore. The heart of it is Olkhon, the largest island, where the twin marble peaks of Shaman Rock rise out of the water at Cape Burkhan. For the Buryat it is one of the holiest places in Siberia, and the ribbons on the wooden poles above it are prayers.
From the middle of February until late March the whole surface is a metre of glass. You walk straight out onto it with the black depth under your boots. Methane off the lakebed is frozen mid-rise into stacked silver bubbles, and pressure buckles the sheet into turquoise hummocks. It is never quiet: the ice booms and cracks all day as it settles. Where waves froze as they hit the cliffs, at Cape Khoboy and the islet of Ogoy, you duck through low mouths into grottoes lit turquoise from inside. Out past the last village lights at twenty below, the sky opens over the glass. In spring the nerpa pup in snow lairs on this ice, eighty to a hundred thousand of them, the only seal on Earth that lives its whole life in fresh water. How their ancestors reached a lake two thousand kilometres from any ocean is still argued over; the likeliest answer is that they came up the rivers from the Arctic in an ice age and stayed.