A typical bathtub holds 40 gallons or so of water. That is 330 pounds. A cubic yard of it, filling what at first glance seems a modest volume of 3 feet by 3 feet by 3 feet, weighs nearly 1,700 pounds, as much as the Smart micro car.And when water is moving at 30 or 40 miles an hour, like the tsunami that inundated northern Japan on Friday, the heaviness of water turns deadly. Imagine 1,700 pounds hitting you at that speed, and each cubic yard of water as another 1,700 pounds bearing down on you. The destructiveness of a tsunami is not just one runaway car, but a fleet of them.     More