One of the breathtaking experiences of travelling in Glacier Bay is to stand on the deck of a ship immediately in front of the face of a tidewater glacier rising almost vertically 200 feet or more into the sky. As you watch and listen, ominous groans and cracks emanate from unseen fractures and small blocks fall, issuing loud whacks as they hit the water below. We wait patiently for something big to happen. Suddenly a tower of ice the size of a 20-story apartment building breaks loose and crashes into the sea. We see it first, then we hear the marvelous sound of "white thunder" that the natives called sumdum. This remarkable phenomenon is repeated again and again, as the river of ice makes its relentless journey from the mountains into the sea.