Complex sentence: Locke continues to doubt that the timer has any meaning, but Desmond assures him that it is all real, as the timer passes zero, Desmond retrieves a failsafe key and crawls underneath the hatch to reach the system termination switch, and inserts the key.
Split into simpler sentences: Locke continues to doubt that the timer has any meaning, but Desmond assures him that it is all real. As the timer passes zero, Desmond retrieves a failsafe key and crawls underneath the hatch to reach the system termination switch, and inserts the key.