At Holme Fen, a nature reserve just south of Peterborough, there is a slightly tired-looking, cast-iron green post. When the Victorians began to drain the nearby lake of Whittlesey Mere, the local landowner, William Wells, sank the post into the peat to measure how much the dirt shrank as it dried out. In 1848 the post’s top was the same level as the earth. Today, its summit is four metres above ground. It marks the lowest point in Britain, and is the centre piece of what is now a stunning silver-birch wood overlooking a man-made lake.