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A 'cave in' or cave-in has several meanings aside from:
A 'cave in' is when a cave, mine or other hollowed out underground structure loses structural integrity and the ceiling collapses. This is a colloquial saying, popularized by the mining industry, or more specifically the miners.
Cave ins are often the most frequent hazard faced by miners in deep shafts. The act of hollowing out significant sections of rock with heavy equipment or explosives often weakens the rock surrounding those sections, making them susceptible to collapse.
This condition can be worsened by a number of factors. In coal mining, for instance, the presence of fossil fuels of one kind can mean the presence of natural gas. Volatile, odorless and explosive methane gas has been a major hazard for coal miners for centuries. Miners can be suffocated due to lack of oxygen, leading to a practice of bringing canaries in mines to indicate presence of gas (because canaries asphyxiate faster). But even more dangerous is the fact that gas can explode and miners must rely on artificial light to see, which in far gone times meant open flames. A gas explosion in a mine means those weakened rock ceilings can collapse. Thus, cave ins are a frequent hazard for coal miners, who must face the possibility of being buried beneath tonnes of rock or, worse yet, trapped behind tonnes of rock to suffocate.
Another factor is water flow making weak rock dangerously capable of collapse. This is more pertinent to spelunkers whose activity of choice is crawling around in cracks and holes created by water flowing over and through limestone. So cave ins can trap and kill cave divers just as easily as miners, though less frequently.
An act of the submissive will
The other major use of 'cave in' is as a verb. To cave in means to bend to someone's will after originally being opposed to that will. For instance, in a store a little boy demands that his mother get him a candy bar, after 5 minutes of his complaining she finally caves in and gets it for him. Note that when used as a verb, cave is conjugated. So it is often colloquially shortened to 'cave.' Which is used thus, 'Well, i, like, put on my sweet face and, like, daddy totally caved and bought me that BMW.' | 
