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Why do submarines sink and rise?

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Answer: Ballast tanks fill or empty

Ballast tanks fill or emptyCorrect! Submarines have special tanks that fill with water to sink (heavier than surrounding water) or fill with air to rise (lighter). This changes buoyancy without changing the submarine's shape or size!

Hull shape changes underwaterWrong. Submarine hulls are rigid and don't change shape. Buoyancy is controlled by ballast tanks.

Water pressure controls depthWrong. Water pressure doesn't control the submarine. The crew controls ballast tanks to change buoyancy.

Go deeper: Ballast tank · Buoyancy
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