A 2018 study of sediment cores taken from the bed of Walden Pond found signs of “cultural eutrophication”: human urine released into the pond since it became a popular swimming spot in the 1920s has altered the water chemistry and could turn the “beautiful clear lake into a slimy green stew.” The study was reported in the Boston Globe with the headline “Please Stop Peeing in Walden Pond, Researchers Beg.”