The Great Ķemeri Bog is made of plants that never finished rotting. Rain falling here has nowhere to go: ridges of old coastal dunes wall off the sea, and dense clay and dolomite below stop it draining away. The plain stays waterlogged, so dead moss piles up faster than it can rot. Over some 8,000 years that moss has built into a layer of peat lying over the sand, 4.8 metres deep on average and eight metres at its deepest. It covers 6,192 hectares, the fifth largest raised bog in Latvia, its surface scattered with hundreds of dark pools, tea-brown with peat. Between them stand pines no taller than you are, many over a century old, kept small by ground that is acidic and almost without nutrients. That same airless ground gave Ķemeri its other export: groundwater dissolves sulphate from gypsum below, and where it meets buried plant matter, bacteria turn it into hydrogen sulphide. That is the sulphur water that paid for the town's first bathhouse in 1838.
A boardwalk runs out from the forest edge: a 1.4-kilometre loop, or a 3.4-kilometre one reaching a tower where the bog opens flat to the horizon. Out on the planks the air smells of wild rosemary and the water sits just beneath the boards. The pools are so still that they mirror the sky almost exactly, which makes the far edge of the bog hard to pick out. Wood sandpipers and golden plovers nest out here, because in Latvia they breed on raised bogs and almost nowhere else, and cranes and geese cross overhead on passage. The bog came through the last century almost untouched; only its eastern edge was drained and cut for peat, until 1973. Those workings have since been re-wetted and replanted with sphagnum, and the waders are moving back in. Protected inside Ķemeri National Park since 1997, it is free and open around the clock, all year; the snow is never cleared, so in winter the boards are often yours alone.