If one digs a deep pit and another comes and broadens it, and an ox falls into it and dies, the rule is as follows: If the ox dies as a result of the asphyxiating atmosphere of the pit, the second person is exempt because his action has rendered the atmosphere less asphyxiating. If, however, it dies as a result of concussion, he is liable since he has increased the likelihood that the pit will cause this damage.
Source: wiktionary