How a Submarine Dives

Watch this submarine. It slides under the surface, sinks down into the blue, and then — it stops. It just hangs there, motionless, in the middle of the water. There's no cable from above. No elevator. Nothing holding it up. So how does a giant steel tube pick exactly the depth it wants, and then simply... stay there?

Remember our tug-of-war? Weight pulls down. The water's up-push pushes up. A normal ship is stuck. Its weight never really changes, so the up-push always wins and it bobs on top forever. It can't choose to sink. But a submarine is sneaky. What if you could change your OWN weight whenever you wanted? Then YOU decide who wins the tug-of-war.

Here's the trick, in a cutaway. Running along the bottom of the sub are big hollow spaces called ballast tanks. On the surface, those tanks are completely full of air. Air is super light — so the whole submarine is light too, and it floats up high, just like a ship. Those tanks are the steering wheel for going up and down. Watch what happens when we let water into them.

To dive, the crew opens vents on the top of the tanks and flood valves on the bottom. Watch — seawater rushes IN through the bottom, and the air it replaces whooshes OUT the top in a big burst of bubbles at the surface. Now the tanks are full of heavy water instead of light air. The sub got heavier. Weight beats up-push, and down it goes, into the deep.

Now for the prized trick. Instead of flooding the tanks all the way, the crew stops at just the right amount. At that exact moment, the sub's weight matches the up-push perfectly. The arrows are the same length. Neither one wins. So the sub doesn't rise and it doesn't sink. It just hovers, weightless, at whatever depth it chose. Sailors call this neutral buoyancy.

So how do you come back up? You blow the water out. The sub carries tanks of compressed air — air squeezed to enormous pressure. Open them, and that air blasts into the ballast tanks, shoving all the seawater out the bottom. Now the tanks are full of light air again. The sub is light. Up-push wins, and the submarine rises toward the surface in a roar of bubbles.

Ballast tanks are the rough control — heavier or lighter. But for fine, precise steering, the sub has little wings called diving planes. When the sub is moving forward, tilting these wings down pushes the nose down, and tilting them up lifts the nose up — exactly like an airplane's wings, but underwater. So the crew uses ballast for the big changes, and the planes to fine-tune the angle and depth as they glide along.

Now you fly it. This slider is how much seawater is in the ballast tanks — all air on the left, all water on the right. Let water in and the sub gets heavy and dives. Blow it back out and it floats up. But somewhere in between is the magic spot, where weight exactly ties the up-push and the sub hangs perfectly still. Can you find it?

So that's the whole secret of the deep. A submarine dives by making itself heavier — letting seawater in. It surfaces by making itself lighter — blowing the water back out. And get the amount just right, and it hovers, weightless, anywhere it likes. But now that it can go up and down... here's the next puzzle. How does anything in the water actually move forward in the first place? That's next: how boats move.