A submarine is basically a giant metal bottle that can sink on purpose — and then come back up whenever it wants. How does a several-thousand-ton hunk of steel decide whether to float or drop like a stone?

The secret is inside: huge tanks called ballast tanks run along the sub's sides and belly. When the sub floats on the surface, those tanks are full of air — and air is incredibly light. The air makes the whole submarine less dense than water, so it floats, just like a beach ball bobs on a pool.

To dive, the crew opens valves at the bottom of the ballast tanks. Ocean water rushes in through the openings, and the air inside gets shoved up and out through vents on top. Water is much heavier than air — so as the tanks fill, the submarine gets denser and denser, heavier than the water around it. Down it goes.

Once the ballast tanks are full of seawater, the sub sinks steadily. The crew uses small horizontal wings called dive planes — like stubby airplane wings on the sub's sides — to steer the angle of descent. Tilt them one way, the sub noses down. Tilt them the other way, it levels off.

When it's time to surface, the sub can't just dump the water out — there's an entire ocean pressing in from outside. Instead, the crew pumps high-pressure air from storage cylinders into the ballast tanks. The air shoves the water back out through the bottom valves, the same way blowing into a straw full of water forces the water out the other end.

As the tanks fill with air again, the submarine becomes lighter than the surrounding water. Physics takes over: the sub rises, slowly at first, then faster, pushed upward by the same buoyancy that floats a cork or a balloon.

Near the surface, the crew slows the ascent with the dive planes, so the sub doesn't pop up like a breaching whale. The top of the hull breaks through the waves gently, water streaming off the sides, and fresh air floods in when they crack the hatches.

So that's the trick: fill the tanks with air to float, flood them with water to sink, and use compressed air to blow the water back out. A submarine is a boat that learned to breathe in and out — turning weight into its superpower.