How does a submarine dive and surface without engines pushing it up or down?. Lay out a step-by-step thread: buoyancy basics, ballast tanks and vents, trim control, and how submarines stay level while moving. Add a mini myth-bust about why they do not simply sink like a rock when they stop.

How does a submarine dive and surface without engines pushing it up or down? The short answer: it changes its buoyancy with ballast tanks, then uses control surfaces to steer the angle. The trick is physics, not a magical underwater elevator.

Step 1: buoyancy. A submarine floats because the water it displaces balances its weight, and it sinks when its overall density becomes greater than the surrounding water.

Step 2: ballast tanks and vents. To dive, crews vent air and flood the main ballast tanks with seawater; to surface, they blow those tanks with compressed air to push the water out.

Step 3: trim control. Depth control tanks and trim tanks make small corrections for salinity, temperature, fuel use, and weight shifts, so the sub stays balanced instead of nosing up or down.

Mini myth-bust: a submarine does not simply sink like a rock when it stops. It can hold neutral buoyancy in trim, and while submerged it can stay level with hydroplanes or stern planes even as it moves forward.