The U-boat was a new kind of war. In 1914 the Royal Navy had no good way to stop a submerged submarine threading the deep-water channel into the Humber - and the Humber led straight to Hull's docks, Grimsby's fish quays, and the rail lines feeding industrial Yorkshire. The Admiralty's answer was concrete. Two round towers, twenty-five metres across, would block the channel between them with a chain-and-net boom strung underwater. The southern fort, Haile Sand, sat at the low-water mark off the Lincolnshire coast. The northern one, Bull Sand, anchored to a sandbank eleven feet below low water - which is to say, the builders worked from a platform built on top of water for years, fighting tides and storms to lay foundations they could not see. Accommodation for two hundred soldiers eventually fitted inside each fort. Started in May 1915, finished in December 1919.

In the Second World War the forts finally got to do the job they were built for. Modernised and reactivated, they hung submerged steel netting across the estuary to deter U-boats heading for Hull's burning docks. The Luftwaffe noticed. Enemy aircraft attacked them regularly - two isolated drums of concrete in open water, impossible to miss yet hard to destroy. The gunners stayed at their posts as the bombs fell, ate their rations, played cards, waited for the next raid. By 1945 the threat had passed. By 1956 the military had given up on them entirely. The forts went on the surplus list and into a long, strange afterlife.

In 1987 Bull Sand Fort was given Grade II listed status. In 1997 the Streetwise Charitable Trust bought it, hoping to convert the structure into a drug rehabilitation centre - the isolation, presumably, was the point. The plan failed. The trust no longer operates. In July 2022 Bull Sand Fort was put up for auction with a guide price of fifty thousand pounds. It sold for four hundred and ninety thousand. Haile Sand Fort, the smaller of the two, had been auctioned in October 2018 and sold for one hundred and seventeen thousand pounds to an unnamed purchaser. The new owners face the same problem the original builders faced: anything you want to do with these forts requires getting concrete, building materials, and water out to a structure in the middle of a tidal estuary, where everything has to be timed against the sea.

From a small aircraft, the forts read as deliberate punctuation in an otherwise empty seascape. Bull Sand sits about four nautical miles offshore on a sandbank that disappears at high tide. Haile Sand sits closer to the Lincolnshire bank, between Cleethorpes and Humberston. Seabirds nest on the gun platforms. Rust streaks the concrete walls. At low water the sandbanks reveal themselves like the shoulders of submerged giants. The forts were built to stop an invasion that never came in 1919, and to fight a war that came twenty years late. They survived both, outlasted their charity-trust owners, and now belong to people who have bought a four-storey concrete drum at sea and will have to decide what to do with it.