Until the 1960s, scholars assumed the Broads were natural features - shallow lakes scooped from the East Anglian lowlands by glaciers or rivers. The geographer Joyce Lambert proved otherwise. Core samples revealed straight-sided walls and squared corners deep in the mud, the unmistakable geometry of human work. Ranworth Broad, like its neighbours, was a medieval industrial site that drowned. The villagers who cut the peat could not have imagined that their excavations would, after sea levels rose in the late Middle Ages, become an internationally protected wildlife sanctuary. The lake holds about a metre and a half of water above the old workings, fringed by reedbed and wet woodland that grew up over the centuries since the diggers walked away.

The Norfolk Wildlife Trust manages Ranworth as part of the Bure Marshes National Nature Reserve, and the floating Broads Wildlife Centre is its public face. From the windows you can watch great crested grebes in their elaborate courtship dances - heads weaving, weed presented like a ceremonial bouquet. Wigeon and gadwall paddle in the shallows. Kingfishers flash electric blue along the reed margins. Cormorants stand on dead branches, wings spread to dry. The centre's location on the water rather than the shore means visitors see what the birds see, more or less at the height where the action happens.

Ranworth Broad carries the full stack of British and international conservation status: Site of Special Scientific Interest, National Nature Reserve, part of the Broadland Ramsar wetland of international importance, Special Protection Area for birds, Special Area of Conservation under European habitats law. The acronyms pile up because the place earns every one of them. The reedbeds host bitterns and marsh harriers. The wet woodland - alder carr, mostly - shelters specialist insects and woodland birds. The open water supports overwintering ducks in nationally significant numbers. Few square kilometres of England carry so dense a layer of legal protection.

The village of Ranworth sits a short walk from the wildlife centre, dominated by the Church of St Helen - known across the region as the Cathedral of the Broads. From the top of its hundred-foot tower, you can see broad after broad spread across the flat country, threaded by the rivers that connect them. On a clear day the Happisburgh lighthouse is visible on the coast, twenty miles east. The view explains the geography in a way no map quite manages: the Broads are not a single lake but a constellation, and Ranworth sits near its centre.