Walden Pond in Massachusetts has been tainted by a host of human activity—including pesticides and urine, according to a new study. Since the mid-20th century tourism boom, swimmers’ urine has created a buildup of phosphorus in the iconic pond that poet Henry David Thoreau once called “green and pellucid.” Over the decades, human activity has affected the quality of its water and increased levels of algae in the pond have made it less clear. Researchers also said that a footpath to Thoreau’s cabin—where he isolated himself for two years—“caused large amounts of soil to wash into the lake,” the study says. Even though a landfill near Walden closed and nearby septic tanks have been upgraded, Thoreau is still at risk of sustaining further damage as a result of climate change. “If we’re not careful, this beautiful, iconic lake … could easily become more like a murky, green stew of algae,” senior researcher Cur Stager told CNET.