Many of us have probably seen the TV documentary offering the theory that ships at sea in places like the Bermuda triangle might be sinking because methane gas released at the sea floor creates bubbles that lower the density of the water, resulting in reduced buoyancy. The theoretical foundation for this is Archimedes principle; the buoyant force is equal to the weight of the supporting fluid that is displaced. If the water is filled with bubbles, which weigh essentially nothing compared to water, the vessel can no longer displace its own weight in the bubbly (less dense) fluid. The piece documents a couple of experiments performed to prove the theory. One involves a small, open, pleasure-craft loaded with bricks floating in open water above a large grid of pipes pumping air into the water to lower the density. A second experiment involves a model ship in a large pool with a similar grid of bubble generators. The model used has open hatch covers that permit the vessel to fill with water if it turns on its side, or water flows over it. I found both of these experiments rather unsatisfying, although the second was more appealing than the first. The small pleasure-craft would not sink when it was surrounded by bubbling water, but it eventually sank when the bow was in more stable water and the open stern was in bubbling water, gradually being filled as water splashed into the boat. There was a noticeable lowering of the back of the boat relative to the waterline that the authors attributed to the lower water density, an effect that became more pronounced as the boat filled with water. The model ship in the pool seemed quite buoyant in spite of the bubbling water until the churning water found its way into the hold. An effective method of sinking the model was the release of a giant bubble directly beneath the ship, effectively creating a "hole" in the water directly beneath the boat for it to fall into and then be overflowed by the mounds of water created around the edges of the bubble. I have no doubt that a giant bubble could engulf a ship, or that a shock wave from an explosion used to create such a bubble could blow a ship apart, or that a ship might suffer structural damage if supported only at both ends if a huge bubble opened beneath its midsection. And I don’t doubt that a very high concentration of bubbles in a fluid will result in reduced buoyancy. But I question the applicability of Archimedes principle to any of these situations. I’d like to hear what other people think about this. What causes an object to float? Fundamentally, it is not the displaced liquid that creates the buoyant force. The displaced liquid is not in contact with the object, and there are no long-range forces like gravity involved that enable the displaced liquid to exert a force on the object. The force that supports the weight of the floating object comes from the collisions of water molecules with the submerged portion of the floating object. On the average, these forces are described as the pressure exerted on the object's surfaces by the fluid. Fundamentally, it is because the pressure at depth in a fluid is greater than the pressure above that a buoyant force exists. If these pressure differences did not exist, there would be no buoyancy. It happens that in a fluid of uniform density the pressure variation with depth depends only on the weight of the fluid in a column between any two depths. The magic of Archimedes principle is that even for complex shapes, the net upward force from the fluid pressure is equal to the weight of the fluid displaced by the object. But what if the fluid is not homogeneous? A thought experiment: Consider a large rectangular tank like the one used in the second experiment in the TV show. Before filling the tank with water, blow up some balloons- lots of balloons- enough balloons to nearly fill the tank. These are special balloons that have no elasticity, so the pressure inside the balloon is the same as the pressure outside. They are not very large balloons, but there are a lot of them. All these balloons are tied with strings to the bottom of the tank in such a way that when the tank is filled with water the balloons will be rather uniformly distributed throughout the liquid. The ones near the bottom of the tank will be smaller than the ones above because the water pressure will compress them. Assume for the sake of discussion that when all is said and done, the volume of water is ½ the volume of the tank, so the average density of the water/balloon combination is only half that of water. Two questions: How does the water pressure vary with depth? Will a block of ice float in this pool? I’ll continue this later- Responses to these questions are welcome.