Complex sentence: Significantly one district extends for four miles out under the North Sea, ventilation was therefore not simple; enough air was needed to ventilate the extremities of the eastern workings, whilst that for the closer faces did not need such a powerful supply.
Split into simpler sentences: Significantly one district extends for four miles out under the North Sea. Ventilation was therefore not simple: enough air was needed to ventilate the extremities of the eastern workings, whilst that for the closer faces did not need such a powerful supply.