In Beowulf, the great Anglo-Saxon epic, the monster Grendel stalks Heorot, Hrothgar’s hall, from his lair in the fens. In the most characteristic tale of England’s past – though set in Denmark it is England’s story – the monster comes from the marsh. The poem itself was likely composed in the kingdom of East Anglia, whose greatest king, Rædwald, was probably interred in the ship mound of Sutton Hoo, and the East Angles knew well the dangers and glamours of marsh and sea.

Think on the map of Britain. There’s probably no outline better known to us today, but it’s a modern creation. Britain, and more specifically England, used to cut a very different profile. The distinction between land and water was not nearly so clear, with vast areas occupying a liminal position between the two, sometimes dry, sometimes wet, according to tide and flood. Great bites into England’s body were made twice a day by the tide, seeping in to the salt marshes and bogs that covered the Fens, pushing the River Thames to half a kilometre wide in the London area, running upstream through Romney Marsh to Bodiam Castle in East Sussex. Names bear witness to this past, with areas, often far inland, being called islands and only habitual use deadening us to the strangeness of the title: the Isle of Thanet at Kent’s south-eastern edge, the Isle of Axholme in Lincolnshire, the Isle of Ely in Cambridgeshire.

Perhaps nowhere is the strangeness of this historic landscape more marked than on the Isle of Thanet. Now firmly part of the mainland, the Wantsum Channel, a tidal watercourse fed by the River Stour, separated the isle from Kent. As the most easterly part of Kent, and with the security of the Wantsum Channel, the Isle of Thanet was the perfect stepping stone for invaders, and they employed it, again and again and again. First, the Romans – Julius Caesar used it as a base in his abortive invasions of 55 and 54 BC – then the Anglo-Saxons, with the legendary Hengist and Horsa being given the isle and liking it so much they decided they wanted the rest of the country too – and, finally, the Vikings: the Wantsum Channel provided safe harbour from fierce Channel storms, and the Northmen first experimented with overwintering in a secure base on the isle before using the tactic to conquer most of England. But the Wantsum Channel, once two miles wide, slowly silted up, although Thanet is still clearly shown as an island in maps into the 15th century. But the slow deposition of silt and the indefatigable drainage work of Augustinian monks sealed the island’s fate, and the last ferry sailed across the narrow strait in 1755. The Isle of Thanet was an island in name only and the Wantsum Channel a drainage ditch: an ignoble end for a piece of history.

The Isle of Thanet’s fate encapsulates much of the difficulties faced by England’s wetland wildernesses. They’re mainly on the east, and when boats were more reliable forms of transport than roads, they became highways for traders and raiders. New ideas and technologies spread easily from the Low Countries to the Low Counties, with Dutch engineers imported in the 17th and 18th centuries to lead the push to drain the flatlands. They were still too wild and too dangerous to be allowed to continue, wet worlds where Parliament’s writ held no sway.

Charles Kingsley saw their end:

A certain sadness is pardonable to one who watches the destruction of a grand natural phenomenon, even though its destruction bring blessings to the human race. Reason and conscience tell us, that it is right and good that the Great Fen should have become, instead of a waste and howling wilderness, a garden of the Lord, where

‘All the land in flowery squares,
Beneath a broad and equal-blowing wind,
Smells of the coming summer.’

And yet the fancy may linger, without blame, over the shining meres, the golden reed-beds, the countless water-fowl, the strange and gaudy insects, the wild nature, the mystery, the majesty–for mystery and majesty there were–which haunted the deep fens for many a hundred years.