Very slow day at work, and the mind has been wandering and wondering... Imagine that you have a semi truck that, in lieu of a trailer full of goods behind it, it has an open-top swimming pool, the length, depth, and width of the trailer. (cool, eh?) Now, imagine that someone is treading water in the center (assuming the trailer is 40 feet long, that would put them 20 feet behind the cab). The truck is moving at, say, 40 miles an hour and hits a concrete wall head-on, bringing it to an instant halt. How badly injured, at all, will the swimmer be? What do you expect would happen? Assume the frame of the truck and trailer is intact. I know that the body of water will continue to move forward and splash over the top of the trailer, probably taking the swimmer with it. But my real question is, will there be any compression of the water? Will the swimmer feel any shock of the impact or will the rapid change of the shape of the water as it swells over the lip be the first indication that something is wrong? What if there is a lid on the trailer, and the swimmer is suspended in the center of the body of water, wearing scuba gear?