In 1870, if you took a spade to the ground in Iowa, or Nebraska, or eastern Kansas, you could push it in to the haft and not hit anything that wasn't soil. Six feet of topsoil. Black, friable, alive. The richest agricultural earth on the planet, by a margin so absurd that European visitors with farming backgrounds went silent when they saw it turned over. Most arable land on Earth carries between one and eight inches of topsoil. The Great Plains carried seventy-two. Nobody had ploughed it. Nobody had fertilised it. Nobody had irrigated it. It had been built, slowly and completely, by something else. Stand back from the spade. Stand back from the field. Stand back far enough to see the continent. A herd of bison, fifty miles wide, takes five days to pass the hillside you are standing on. Colonel Dodge recorded this in Arkansas in 1871, and he was not the only one. From the top of Pawnee Rock the herd ran to the horizon in every direction at once. The earth, observers wrote, trembled at three miles. Sixty million animals. The largest gathering of large mammals the planet has ever held. They had been doing this for ten thousand years. The grass grew tall because the bison grazed it hard and moved on. Their hooves broke the crust for seed. Their wallows held the rain. Their dung fed the microbes. Their carcasses fed them harder. The deep-rooted prairie grasses, big bluestem, switchgrass, Indian grass, drove their roots fifteen feet down, locking carbon into the soil at a depth no plough would ever reach. The bison built the six feet of black earth. The bison were why it existed. Then the hide market arrived. Five thousand bison a day, shot from train windows, left to rot. The U.S. government encouraged it openly, because starving the Plains nations was cheaper than fighting them. By 1889, of the sixty million, five hundred and forty-one remained. The plough followed within a decade. The grass was turned under. The hooves and the wallows and the dung had stopped. The soil, untethered from the system that built it, dried. In April 1935 it rose into the sky as a black wall a thousand miles wide and travelled to the Atlantic. Six feet of soil, built over ten millennia, blown into the sea in a generation. There is no putting the bison back at that scale. The cow is the closest analogue the continent has. Run her like a bison, on grass, on the move, in a tight mob. Watch what the land does.