When night falls over a drowned quarry, hope crawls out of despair. The human swimmers who play by day here, sploshing back and forth under sheer limestone walls, will not see it. Teams of wetsuited divers will not either, and they have other priorities. These divers go deep, sinking 25 metres to the rock bottom, where wheelhouse wrecks of boats are metal boxes to discover and day becomes darkness, lit fitfully by torch beams.

Standing in the sunlit uplands of a grass bank, looking down through clear blue-green water to submerged stone outcrops, our impossible quest is to see nocturnal creatures of cracks and cavities. This giant pool is a small sanctuary for breeding native white-clawed crayfish, hand-width crustaceans which have been all but wiped out by invaders in a changed water world outside. Our guides call this place an ark, in a determined belief that, one day, white-claws will reclaim a future beyond.

‘This giant pool is a small sanctuary for breeding native white-clawed crayfish.’ Photograph: Sarah Niemann

One by one, we pull on the strings that draw artificial refuge traps up from deep below. These are racks of tubular black pipes in different diameters with one end open, each designed to suit crayfish large, medium or small. The animals shelter here by day, squeezing in hermit-crab-like, tail first. The fished-out traps have caught silt, sticks, waterweed and zebra mussels, but crayfish there are none. Until the very last chamber.

She plops into an orange bucket, lobster-petite, and quickly sets off to explore. Her dainty pincers are more nail clippers than bolt cutters, held either side of her snout, nudging forward alternately almost piston-like. Four pairs of legs behind give her a scuttle-shuffle motion, until expert Jen lifts her by the abdomen between thumb and forefinger for measurement by callipers. She is the most docile and obliging of animals, even when she is turned over to be sexed. Not yet ready to breed, shell about to be shed.

Later today, in the river close by, we will find snapping furies, the American signal crayfish that are bigger, predatory, and prolific breeders; carriers but not catchers of disease. But here at least, she is safe, secure in her ark.