Complex sentence: Concrete tunnels were used to deliver water and steam to all the buildings; between the nun's villa and the sanatorium, the tunnel was large enough to be used by employees during the hard winters when the temperature often drops below forty degrees.
Split into simpler sentences: Concrete tunnels were used to deliver water and steam to all the buildings. Between the nun's villa and the sanatorium, the tunnel was large enough to be used by employees during the hard winters when the temperature often drops below forty degrees.