The Fens are very low-lying compared with the chalk and limestone uplands that surround them – in most places no more than 10 m above sea level. Indeed, as a result of drainage and the subsequent shrinkage of the peat fens, many parts of the Fens now lie below the average sea level. Although one writer in the seventeenth century described the Fenland as all lying above sea level (in contrast to the Netherlands), the area is now home to the lowest land point in the United Kingdom, Holme Fen in Cambridgeshire, at around 2.75 metres below sea level. Within the Fens there are a few hills, which have historically been called "islands" as they remained dry when the low-lying fens around them were flooded. The largest of the fen-islands is the Isle of Ely, on which the cathedral city of Ely was built; its highest point is 39 m above mean sea-level.