Right. I didn't mean to imply that the water is only under the U.S. I meant to say that under the U.S. there is 3 times the volume of the world's oceans. That seems like an awfully large amount of water for an area that comprises around 2% of the Earth's surface. If this rock wraps around the entire Earth evenly, that suggests that there is 150 times the volume of water of all the Earth's oceans 700 kilometers down. The idea that the oceans, which cover 70% of the surface and may be miles, deep only make up .007 of the volume of water on Earth blows my mind.

EDIT: 2% = 1/50th of the surface. 3-oceans * 50 = 150. (1 * all earth's oceans) / (150 * all earth's oceans) =~ 0.00666...