A study by the Byrd Centre, published in the Proceedings of the National Academy of Sciences, indicates that the burning of coal in Europe during the Industrial Revolution has led to metals being deposited and trapped in the Dasuopu glacier in the central Himalayas. Dasuopu, at 23,600ft above sea level, is the highest-altitude site in the world where scientists have obtained a climate record from an ice core. A team travelled in 1997 to drill ice cores, so as to study layers of snow, atmospheric circulation and other environmental changes over time. The study proved that the snow had higher-than-natural levels of toxic metals, including cadmium, chromium, nickel and zinc. These metals are by-products released when coal is burnt. It is believed that the metals were most likely transported by winter winds, which travel from the west to the east. Scientists also believe that zinc could have accumulated due to large-scale forest fires. According to the scientists, the contamination in the ice core was most intense from about 1810 to 1880.