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Why do submarines sink and rise?

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Answer: Ballast tanks fill or empty

Ballast tanks fill or empty ✓ — Correct! 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 underwater — Wrong. Submarine hulls are rigid and don't change shape. Buoyancy is controlled by ballast tanks.

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

Go deeper: Ballast tank · Buoyancy
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