Lake Baikal

Polly woke up on the fifth morning and the world had become water.

The train was running along the southern shore of Lake Baikal. The shore was a thin band of beach and forest between the tracks and the water. The water itself filled the entire window from edge to edge: not a lake so much as a sea, except for the perfect stillness and the impossible clarity of the colour, which was a blue that seemed to start from somewhere deep below the surface and rise up.

The fisherman, whose name turned out to be Igor, had been awake for two hours. He was at the window with a small notebook, writing something Polly could not see. He turned when she perched on the rail beside him.

"Baikal," he said. "Have you read about it?"

Polly tilted her head.

"It is the deepest lake in the world," he said. "One thousand six hundred and forty-two metres at its deepest point. The deepest freshwater on Earth. It is also the oldest lake. Twenty-five million years old. Most lakes fill in with sediment over time. Baikal is in a rift valley that is still growing. It gets a few centimetres deeper every year. The bottom drops faster than the sediment can fill it."

The lake outside the window was so clear that, looking down from the moving train, Polly could see boulders on the lake floor as the shoreline shallowed. The clarity is up to forty metres in the right conditions, Igor said. In the deep middle of the lake, the water is the cleanest fresh water on the planet.

"It also holds," Igor said, almost as if reciting, "twenty-three percent of all the unfrozen fresh water on Earth. More than all the North American Great Lakes combined. The water in Baikal alone, if it were to be drained, would supply every human on Earth with fifty years of drinking water."

Polly looked at the lake. It did not look like twenty-three percent of anything. It looked like a single still blue thing.

The train stopped at a station called Slyudyanka. A small Russian Orthodox church was visible from the platform. Igor got off. He shook Polly's wing tip with two fingers, gravely. "Three days fishing," he said. "Then home." He waved through the window once and was gone.

The train pulled out along the lake. For three more hours, it ran the shore. Polly stayed at the window the whole time. The trees on the slope above the tracks were Siberian pine and larch. A small wooden chapel passed. A solitary fisherman waded in the shallows in green rubber boots. A group of seals, Baikal seals, popped their heads above the surface a kilometre offshore. They are the world's only purely freshwater seal species. They are found nowhere else.

Polly thought about scale. Half a kilometre below her, at this exact moment, was water older than her species. Below that, more water, and below that, more water, all the way down to a depth that would not be illuminated even at full noon. Inside that water was a tiny pink shrimp called Epischura that did not exist anywhere else on Earth, and which functioned, collectively, as the lake's filtration system. They kept the water clear. They had been keeping it clear for twenty million years.

The train ran. The water held still. The day passed.