Large ocean/sea mammals, say a blue whale, are able to withstand large water pressures underwater, and I read that they're able to do so for a number of reasons, one of which is that the air cavities are thought to fill with water. Is it this equalization of pressure that allows whales to deep dive, or is it the absence of air in its cavities? There is a distinct difference between these two ideas, though similar. So, say the whale equalizes pressure to survive deep dives. The air cavity is displaced by water. There is an absence of air because there is now water. But conversely, let's say that you have an unmanned submarine that has a vacuum inside equal to zero air pressure or 0 ATM. Would the lack of the presence of air be the primary reason that deep dives are possible, all things constant (e.g. the sub's hull can withstand external water pressures of the deepest dives)? If there is no air inside to be affected by pressure, would a vacuumed object be able to withstand greater water pressures? Or is it something else? Thanks in advance.