I was fascinated by the Wired article describing such an elegant engineering solution for remote data collection deep in the ocean, Deep Glider: http://www.wired.com/news/technology/0,72794-0.html?tw=rss.index But, having been out of engineering school a few decades and a bit rusty in my physics and hydrostatics, I was intrigued by a statement in the article: "Traditional gliders consume about half a watt of energy moving at a rate of half a knot. Deepglider's power consumption is about half that because of its exceptionally stiff hull that's resistant to pressure. When pressure compresses a hull in a traditional glider, it gains buoyancy and requires more energy to control." I always thought bouyancy was the relative upward force of water on an object equal to the weight of water displaced by the object. Ignoring the change in water density (since water is most dense at 4º C usually the temperature at depth is I recall from a Blue Planet video), assuming the glider's mass remains constant, and ignoring the pressure difference over the vertical dimension of the glider since it is quite short, then as the hull descends and is compressed its volume decreases. This reduced volume displaces less water reducing the bouyant force on the glider making it less bouyant, not more so. Still perhaps harder to control but for the opposite reason. Am I wrong in my cursory analysis drawing on a long-dormant physics and engineering education?