Perito Moreno Glacier, Santa Cruz, Argentina. December 14th, 2011.
This is the point where the glacier crosses the Rico arm of the Lago Argentino and reaches the other shore, forming a natural dam on the lake. This causes the water level on the lake to begin rising up to 30 meters. Eventually the pressure of the water causes it to seep under the glacier, resuming circulation to the other side. The water then begins to erode the glacier, until it forms an enormous arch that eventually collapses in spectacular fashion. This happens every 4 or 5 years, bringing people from all over the world to watch it.