A submarine can descend or rise by changing its overall density relative to the water around it. It does this by filling its ballast tanks either with water (increasing its density) or with air (decreasing its density). Decreasing the amount of water in its ballast tanks by replacing it with air lowers the total mass of the submarine; this decreases its density. The picture at the right shows a simplified cut-away view of a submarine; you can see the ballast tanks. Compressed air from storage is vented into the tanks, pushing the water out the valves underneath. The ballast tanks full of air make the submarine less dense, so it rises. To descend, the bottom valves allow water to enter the ballast tanks, and the air escapes from valves on top; as the tanks fill with water, the sub again becomes more dense relative to the water around it, so it descends. Watch the animation below and you can see the whole process in action. Once the submarine reaches the depth it wants by flooding the ballast tanks, some air is blown back into them to adjust the buoyancy so that the submarine's overall density is the same as the water around it; this allows it to remain at that depth. While it is underwater, allowing more water into the ballast tanks will enable it to go lower; the displaced air is captured and recompressed to use again.This also means that no bubbles (or noise) will give away the sub's location. Resources Content, HTML, graphics & design by Bill Willis 2023