The 5 kilometre-long Tuen Mun-Chek Lap Kok Link twin-tube tunnel will link the New Territories in the north to the artificial island of Hong Kong-Zhuhai-Macao Bridge Hong Kong Port, providing faster access to the international airport. The world's largest slurry tunnel boring machine (with a diameter of 17.63 metres) was used to drive an initial 600-metre-long section before being converted for the section of tunnel that narrows from three lanes to two. Once the shield and cutter head of the first TBM had been changed, two 14-metre diameter machines advanced within a high-pressure environment, excavating 4.2 kilometres of subsea tunnels. With excavating areas reaching up to 50 metres below sea level, maintenance operations were carried out in hyperbaric conditions, by teams of specialised divers living at the surface in pressurised habitats.