Listen carefully: Surprisingly often, massive icebergs calve off the 200-foot face of Moreno Glacier, in southern Argentine Patagonia, and crash into the water. Here, a low pass in the Andes lets Pacific storms cross the cordillera, and countless storms have deposited mountains of snow that, over the millennia, have compressed into this flowing river of ice. Several times in this century, the advancing glacier has blocked a channel to form a rising body of water that eventually, when the water's weight became too much for the dam, triggered an eruption of ice and water toward the glacial trough of Lago Argentino. Today, you can spend entire days either gazing at or simply listening to this awesome river of ice as it rumbles forward.