WordNet (r) 3.0 (2006):

cave in
    n 1: the sudden collapse of something into a hollow beneath it
         [syn: cave in, subsidence]
    v 1: break down, literally or metaphorically; "The wall
         collapsed"; "The business collapsed"; "The dam broke"; "The
         roof collapsed"; "The wall gave in"; "The roof finally gave
         under the weight of the ice" [syn: collapse, fall in,
         cave in, give, give way, break, founder]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "cave in":
   back down, bowl, breach, break, break down, break in, break into,
   break off combat, break open, break through, breakdown,
   breaking up, breakup, burst in, bust in, cataclysm, catastrophe,
   cave, cease resistance, collapse, come apart, come unstuck,
   concave, conk out, crack-up, crash, crumble, cup, debacle, decline,
   deflate, deflation, disaster, dish, disintegrate, droop, drop,
   fade, fail, faint, fall in, fizzle out, flag, flop, flop down,
   flump, flump down, fold, fold up, force open, founder, give ground,
   give in, give out, give up, give way, go down, go downhill,
   go soft, go to pieces, hit the skids, hollow, hollow out, implode,
   implosion, languish, lapse, lower, peg out, peter out, pine, plop,
   plop down, plump, poop out, prize open, prostration, puncture,
   quit the field, rupture, sag, set, settle, settle down, shipwreck,
   sink, sink down, slouch, slump, slump down, smash, smashup,
   split open, stove in, submerge, subside, succumb, swag, sway,
   tear open, topple, topple down, topple over, total loss, totter,
   washout, weaken, wear away, wear thin, wilt, wrack, wreck, yield