Complex sentence: There are two additional shafts leading to the surface, one with dimensions 3.5 m x 3.5 m (45 m) and one with diameter 0.5 m (40 m), presently used to supply electricity.
Split into simpler sentences: There are two additional shafts leading to the surface. One with dimensions 3,5 m x 3,5 m (45 m) and one with diameter 0.5 m (40 m), presently used to supply electricity.