BC Hydro's Cathedral Square substation is a major 230/12 kV underground station in the heart of downtown Vancouver. The civil design of the substation, including site investigation, excavation, structural design and construction, is described. In addition, the mechanical design is described covering fire protection systems, station drainage, sewage and water supply, cooling systems and heating ventilation and air-conditioning and a unique transformer handling system. The electrical design of the substation is the subject of two companion papers.