Under HATS Stage 1, a total of 23.6 kilometres of tunnels are constructed for conveying sewage from various parts of Kowloon and north-east Hong Kong Island to the Stonecutters Island Sewage Treatment Works. These tunnels are of bored diameter 2.6 to 5.6 metres, and located 76 to 150 metres below ground level. Most of these tunnels are drilled by Tunnel Boring Machines (TBM) which can cut through sound rock.