Complex sentence: This was intended to be used as the bomb-proof outlet for a ventilation and air conditioning shaft--an essential element for a facility where dangerous and explosive gases were expected to be used in large quantities on a daily basis.
Split into simpler sentences: This was intended to be used as the bomb-proof outlet for a ventilation and air conditioning shaft. It was an essential component of a facility where dangerous and explosive gases were expected to be used in large quantities on a daily basis.