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The largest canyon on land today is the Grand Canyon, one of the seven natural wonders of the world, 446 kilometers long, 29 kilometers wide, and 1,857 meters deep, that we reviewed in Chapter 1. Of course, these dimensions pale compared to oceanic basins, but despite their far more extensive dimensions, oceans are always filled with seawater. Or are they? As we have seen in the chapter on plate tectonics, oceans do not last forever.