Tourists wait to see the rupture of the leading edge of the Perito Moreno glacier near the city of El Calafate in the Patagonian province of Santa Cruz, southern Argentina, March 2. 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 comes tumbling down. The phenomenon repeats itself at irregular intervals, with the last major ice falls occurring in 2008. The glacier collapsed on March 4, 2012, after several large chunks broke off.

The Perito Moreno Glacier in Los Glaciares National Park, in southern Argentina, completed its break in the middle of a storm when the darkness and rain conspired to frighten hundreds of tourists waiting anxiously for the show on March 4. The breakdown process, which began on Feb. 29, was caused by the pressure of water above the ice dam, which began to crack to form an arch-shaped hole that ended up weakening and finally collapsing. The natural phenomenon happens infrequently -- from once a year to less then once a decade.

The Perito Moreno glacier is seen after the rupture of a massive ice wall near the city of El Calafate in the Patagonian province of Santa Cruz, southern Argentina, March 4, 2012. The glacier, a massive tongue of ice in the Santa Cruz province that covers 250 square kilometres (97 square miles), advances yearly into a lake, known as Lago Argentino.