Lake Baikal is the deepest and oldest lake on Earth, a crescent 636 kilometres long in the mountains of southern Siberia, and it holds about a fifth of all the unfrozen fresh water on the planet, more than the five North American Great Lakes together. It is deep because it is a wound: the lake fills a rift where two slabs of the Earth's crust are slowly tearing apart, 1,642 metres to the bottom, and it has been filling for some 25 million years, long enough to grow its own animals. The strangest is the nerpa, the only seal on Earth that lives its whole life in fresh water, some 80,000 to 100,000 of them, hauling out on offshore rocks in summer and on the ice in spring. The water is so clear that a white disc can be seen forty metres down, and in summer boats run along the wilder shores, past sheer cliffs dropping into it, hidden coves and pillars of rock standing off the beach.
Winter is when Baikal becomes something no other lake is. From mid-February to late March the whole surface hardens into a metre-thick sheet of ice as transparent as glass, and you walk straight out onto it with the black depth under your boots, over methane bubbles from the lakebed frozen mid-rise into stacked silver columns and pressure ridges buckled up in turquoise slabs. Where waves froze as they struck the cliffs, the shoreline is hung with ice caves and icicles, best around Cape Khoboy and the islet of Ogoy. The heart of the lake is Olkhon, its largest island, and on Olkhon's Cape Burkhan the twin marble peaks of Shaman Rock rise straight out of the water; for the Buryat people it is one of the holiest places in Siberia, where only shamans could once perform their rites, and the ribbons tied to the posts above it are prayers. Baikal is a lake the size of a sea, older than most of the mountains around it, and in winter it lets you walk on the deepest water on Earth.