In 2015 the president of the Nepal Mountaineering Association warned that pollution, especially human waste, has reached critical levels. As much as "26,500 pounds of human excrement" each season is left behind on the mountain. Human waste is strewn across the verges of the route to the summit, making the four sleeping areas on the route up Everest's south side minefields of human excrement. Climbers above Base Camp—for the 62-year history of climbing on the mountain—have most commonly either buried their excrement in holes they dug by hand in the snow, or slung it into crevasses, or simply defecated wherever convenient, often within meters of their tents. The only place where climbers can defecate without worrying about contaminating the mountain is Base Camp. At approximately 18,000 feet, Base Camp sees the most activity of all camps on Everest because climbers acclimate and rest there. In the late-1990s, expeditions began using toilets that they fashioned from blue plastic 50-gallon barrels fitted with a toilet seat and enclosed.