The lowest point of land in Britain is a cast-iron post in a Cambridgeshire wood, and when it went in the ground it was level with the ground. Stand next to it now and it is four metres over your head. It is at Holme Fen, south of Peterborough, and in 1851 this was the edge of Whittlesey Mere, the largest lake in southern England. A syndicate of landowners drained the lake that year for farmland, and the engineer, who knew what happens to peat when you take the water out of it, drove an oak pile through 22 feet of it down to the clay and cut the top flush with the turf. A column from the Crystal Palace replaced it. Then they stood back and watched the land go down. Nine inches in the first year. Eight feet by the 1890s. Today the ground around that post is 2.75 metres below sea level, and the post is a ruler nobody asked for, sticking out of a birch wood with guy ropes holding it up because the soil that used to hold it up has gone into the sky. That is what drained peat does. It dries, it shrinks, and it oxidises, which means it turns into carbon dioxide and leaves. Peat takes a thousand years to lay down a metre. Under the plough it loses a centimetre or two of that every year, and the Fens have been under the plough since the Dutch engineers arrived in the 1600s. Peat soils farmed for crops are among the largest sources of carbon dioxide in Britain's whole land sector, and the fields doing it are growing lettuce. Because that is what the Fens are for. A third of England's vegetables come out of this one flat black square: half the lettuce grown in Britain, three quarters of the celery, the carrots, the onions, the potatoes, the leeks, the beetroot. The bag of salad, the celery sticks, the chips. Every field of it is a few millimetres lower than it was last January, the pumps run day and night to keep the sea out of a landscape that is now beneath it, and the water the crops need is forecast to run short within ten years. Before the drain this was eels, wildfowl, reeds, summer grazing, and a fen that had been quietly stacking carbon since the ice left. The cattle came and went on it for 5,000 years and the ground stayed where it was. The salad has been on it for 170 years and the ground is four metres down. The post is still measuring.