A worm the length of your forearm. Not the small, pink worms that wiggle in the garden – the ones you dig up when planting tomatoes. A worm with a body as thick as your thumb, as long as a ruler, as muscular as a snake. That was the giant intertidal worm, and it was the sand ghost of the tidal flat. Size and its digging made it special. A fifty-centimetre worm is a formidable burrower. It can move through the sediment like a submarine through water, swallowing sand, digesting the organic matter, and excreting the clean sand behind it. It is the plough of the estuary, the one that turns the mud and keeps the sediment aerated. It lived in the intertidal zone – the places where the tide rises and falls twice a day. It dug a deep, J-shaped burrow in the sand or mud, with a shaft down, a horizontal chamber, and a second shaft up. It fed by swallowing sand from the surface, digesting the organic matter, and excreting the clean sand in a coiled cast at the entrance of its burrow. It ploughed. It aerated. It recycled. The giant intertidal worm was a bioturbator – an animal that mixes and aerates the sediment. Its burrowing brought oxygen into the mud, allowed water to percolate through the sand, and created habitat for other small creatures. It was a keystone species of the tidal flat, the one that kept the sediment healthy. Lugworms reproduce by releasing eggs and sperm into the water. The larvae drift in the plankton for weeks before settling on a tidal flat and digging their first burrow. That strategy works when the flats are vast and the water is clean. It fails when the flats are drained and the water is polluted. Coastal development and harbour dredging destroyed it. When Europeans arrived, they drained the tidal flats for farmland. They dredged the harbours for shipping channels. They filled the bays for housing. The giant intertidal worm, which needed vast, undisturbed tidal flats to survive, lost its home. At the same time, pollution from farms, towns, and industry poisoned the sediment. The giant intertidal worm, which swallowed sand by the mouthful, ingested the toxins with its food. It could not survive. By the 1900s, it was gone. The last specimens were probably collected by a naturalist who had no idea he was holding the final individual. He preserved them in alcohol, labelled them, put them in a drawer. And the tidal flats fell silent. The smaller worms survived. The common lugworm is still present in our remaining tidal flats – smaller, faster, more adaptable. It is the survivor, the one that kept its head down. But the giant intertidal worm is extinct. A few specimens in a museum jar, a few fragments of its DNA, and the memory of a worm that used to plough the tidal flats, a sand ghost in the mud. The sand ghost has faded. The mud is not as healthy as it used to be.