The collapse of the Perito Moreno glacier near the city of El Calafate in the Patagonian, southern Argentina, attracts thousands of tourists. This year, the leading edge of the glacier collapsed March 4. As Perito Moreno moves forward, it cuts off a river feeding the lake. Water builds up pressure and slowly undermines the ice, forming a tunnel until ice sheets come tumbling down. The phenomenon repeats itself at irregular intervals. The last before the March 4 collapse, was in 2008.