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 lake filled the entire window: 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 somewhere deep below the surface and rise up.

Igor, the fisherman, had been awake for two hours. He was at the window with a small notebook.

"Baikal," he said. "It is the deepest lake in the world. One thousand six hundred and forty-two metres at its deepest point. 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. The bottom drops faster than the sediment can fill it."

The water outside was so clear that Polly could see boulders on the lake floor in the shallow areas. The clarity is up to forty metres in the right conditions.

"It also holds," Igor said, "twenty-three percent of all the unfrozen fresh water on Earth. More than all the North American Great Lakes combined."

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 Slyudyanka station. Igor got off. He shook Polly's wing tip with two fingers, gravely. "Three days fishing. Then home." He waved through the window and was gone.

For three more hours, the train ran the shore. A group of Baikal seals popped their heads above the surface a kilometre offshore. They are the world's only purely freshwater seal species.

Polly thought about scale. Below her was water older than her species. Inside that water was a tiny pink shrimp called Epischura that did not exist anywhere else on Earth. It kept the lake clear. It had been keeping it clear for twenty million years.