cave in

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The act of something collapsing or caving in.
• noun The location where something has caved in.
• noun idiomatic The act of relenting.
• verb To collapse inward or downward.
• verb idiomatic To relent; to acquiesce; to comply; to grant approval; to lose willpower.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the sudden collapse of something into a hollow beneath it
• verb break down, literally or metaphorically