In 1699 Dummer adapted the channel leading to the Upper Wet Dock, enabling it to be closed off at each end by a set of gates, thus forming a second dry dock (called the "North Stone Dock" after it was rebuilt with stone altars in 1737, and known today as No 6 dock). Severed from the harbour, the Upper Wet Dock became a reservoir into which water from various nearby dry docks could be drained; it was vaulted and covered over at the end of the eighteenth century, but still exists today underground. By 1700 a shipbuilding slip had been constructed off the (Lower) Wet Dock, parallel with the dry dock (roughly where No 4 dry dock is today).