Bawsey pits were dug to fuel the building boom, as King's Lynn grew in the 1960s and 70s.

The former sand workings are part of a chain of flooded pits and quarries which stretch west from King's Lynn.

Once the seam of sand or gravel below the water table is exhausted, or when it becomes too expensive to pump water clear, the workings are abandoned to nature.

Some lakes in the area are tucked away down farm tracks, away from the public gaze, known only to anglers and bird watchers. Others have become holiday parks, sailing clubs or fishing syndicates.

Owner Sibelco opens Bawsey Pits to the public, but signs around the lakes forbid swimming. Depths are unpredictable where gravel has been dug by drag line, leaving bars and much deeper areas where the water can plunge to 20ft or more.

Swimmers risk cramp if they unexpectedly enter a colder pocket of water, the legs literally seizing up in the cold.

But the Pits have become an alternative to travelling to the coast for many. On a sunny day, hundreds of families pitch up beside the waters. Many ignore the signs which forbid swimming.