Synonyms for Plunge:

n.

dwindle, shrink, decrease, crater, diminish. back up, culvert, cesspit, airlock, clog, conduit, cutoff, wade in, downspout, blockage, enter, lunge. belly flop, move, the butterfly, decompression, backstroke, forge, breaststroke, crawl, bathing, cannonball. lose your balance/footing. come under, be in dire/desperate straits, suffer, undergo, incur, not know where/which way to turn, go through, be in the grip of something, endure. cast off, hydroplane, heave to, ground, capsize, heel, dock, go about, come about, founder. stumble, landing, trip, pratfall. act (noun)
dip.
descent (noun)
descent, comedown, grade, slope.
plunge (noun)
douse, dip, engulf, engross, dive, soak up, plunk, launch, souse, swoop, pitch, steep, immerse, absorb, dunk.
swan dive (noun)
cannonball.

v.

descend (verb)
descend, drop, come down, topple, lower, fall down, fall, settle, decline, slide, slip, tumble, plummet, slump.
dive or fall fast (verb)
descend, pitch, dip, tear, topple, go down, rush, submerge, plummet, dash, tumble, plunk, drop, immerse, lunge, drive, thrust, sink, charge.
immerse (verb)
dip, flood, drown, bathe, dunk, engulf, immerse, wallow, submerge, douse, inundate, sink.
lower (verb)
shorten, deepen, depress.
plunge (verb)
swoop, dive, pitch, fall, plummet, drop.
run (verb)
dash, rush, canter, lope, scoot, scurry, race, dart, hurry, gallop, bound, sprint, run, jog, tear, trot, scramble, scamper.

Other synonyms:

hasten, cutback, reduction, shrinkage. stumble, pratfall, lunge, collapse, nose dive. ram, decrease, skid, spill. thrust. landing. stab, dig, drive. trip. go down. cut. cascade
flood.
descend
stumble.
hasten
hasten.
invest
put in.

Usage examples for plunge

  1. Arnold made another plunge – Man and Wife by Wilkie Collins
  2. I heard a single throb of the machinery, and the plunge was made, the submarine disappeared beneath the waters of the lake. – The Master of the World by Jules Verne
  3. She liked handsome town- houses, and she was sure she would like society; but it would all be very new and strange to her, and, although she was a brave girl at heart, she shrank from making such a plunge as this. – The Late Mrs. Null by Frank Richard Stockton