plummet 强调垂直，快速
plunge 可用于价值下降，坠落
以下是字典中的解释
plummet
verb (plummeted, plummeting )
[no obj.]fall or drop straight down at high speed
快速落下，陡直落下
a climber was killed when he plummeted 300 feet down an icy gully.
一位登山者从300英尺高处陡然掉到一个冰谷里给摔死了。
■decrease rapidly in value or amount.
（在价值或数量上）骤然下跌，陡然下降
hardware sales plummeted.
硬件销售量陡然下降。
noun
a steep and rapid fall or drop
骤跌，陡跌，骤降
plunge [pl?nd?]
verb
[no obj., with adverbial]jump or dive quickly and energetically
（很快地有力地）跳入，扎进，投入
our little daughters whooped as they plunged into the sea.
我们的小女儿们欢呼着跳进了大海。
■fall suddenly and uncontrollably.
（突然失控地）一头坠入
a car swerved to avoid a bus and plunged into a ravine.
一辆小车急转方向以避开一辆公***汽车，因而一头坠入了峡谷之中。
■suffer a rapid decrease in value.
（价值）猛跌
shares in the company plunged 18p on news that profits had fallen.
利润下降的消息传来，公司的股票便骤然猛跌了18便士。
n.
投身入水
John felt refreshed after a quick plunge in the lake.
在湖中游了一会儿泳后, 约翰感到精神振奋。
猛跌, 骤降
The sky diver had a plunge of more than 10, 000 feet before his parachute opened.
那个跳伞运动员坠落了一万多英尺, 他的降落伞才打开。