Diving or falling steeply downward with force; or moving quickly into a situation.
From Old French 'plongier,' possibly from Latin 'plumbum' (lead), imagining something falling like a weight. The sense of rapid descent is primary.
The Mariana Trench represents an extreme plunging dive—if you could somehow fall through 11 kilometers of ocean, the water pressure would be 1,000 times stronger than at the surface.
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