[intransitive, transitive] to move or make somebody/something move suddenly forwards and/or downwards
+ adv./prep. She lost her balance and plunged 100 feet to her death.
The train left the track and plunged down the embankment.
plunge somebody/something + adv./prep. The earthquake plunged entire towns over the edge of the cliffs.
[intransitive] (of prices, temperatures, etc.) to decrease suddenly and quickly
synonym plummet
Stock markets plunged at the news of the coup.
This year profits plunged by 40%.
[intransitive] + adv./prep. (of a road, surface, etc.) to slope down steeply
The track plunged down into the valley.
[intransitive] to move up and down suddenly and violently
The horse plunged and reared.
(figurative) His heart plunged (= because of a strong emotion).