When does a submarine....a)sinkb)floatC)rise
All three states (floating, sinking, rising) have to do with buoyancy, or the tendency of an object to float within another. Being "neutrally buoyant" means that the object (in this case, a sub) exactly displaces its weight in the water (in other words, once the sub is launched into the ocean, an amount of water weighing exactly the weight of the sub is displaced) and so will remain at a given depth, neither sinking or rising. Submarines possess ballast tanks, which contain either water, air, or both. At neutral buoyancy, the tanks contain water and air. Adding more compressed air to remove the water ("blowing the tanks") reduces the overall weight of the sub, so it will rise; if enough air is added it'll reach the surface and float. Adding more water and removing the air increases the weight, so the sub will sink. Fish possess an "air bladder" which enables them to do the same thing. By regulating the amount of air, the fish sinks, rises, or floats within the water.