Lough Neagh is the largest lake in the British Isles. It supplies more than two-fifths of Northern Ireland's drinking water. And for three summers running it has been too poisonous to touch. The whole lake turns a livid, paint-thick green, smothered under toxic blue-green algae. Bathing signs line all seventy-eight miles of shore. Dogs that drank at the edge fell sick. Europe's largest eel fishery, worked for centuries, was shut down. Scientists pulled the scum apart and found it teeming with hazardous bacteria, E. coli and salmonella among them, plus a toxin never before recorded on the island of Ireland. And they are still drawing drinking water out of it. The lake is now classed hypereutrophic, the worst grade of nutrient pollution there is. The poison is phosphorus, which feeds algae like rocket fuel, and the official figures lay around 62% of it at the door of agriculture, with roughly another quarter from sewage. None of this was a mystery. A field grazed at a sensible rate hands its nutrients back to the soil that fed it. Cram in far more animals and far more chemical fertiliser than the ground can absorb, and the surplus pours off into the rivers and down into the lough. Northern Ireland runs that phosphorus surplus year after year, because a government growth strategy told farmers to expand and keep expanding. Campaigners warned for decades exactly where it would end. They were met with shrugs. So the largest lake in these islands was knowingly fed to death to chase agri-food output, and a warming climate and an invasive mussel finished what the slurry started. The cruelty is the timeline. Stop every drop of pollution tomorrow, and the phosphorus already buried in the sediment will keep the blooms coming for up to forty years. A lake ten thousand years in the making, wrecked in a single generation, and the people who run it are still pulling the tap water out of the green.