The second half of the eighteenth century was a key period in the development of Portsmouth (and indeed of the other Royal Dockyards). A substantial planned programme of expansion and modernisation was undertaken from 1761 onwards, driven (as would be future periods of expansion) by increases both in the size of individual ships and in the overall size of the fleet. In the 1760s the Lower Wet Dock (by then known as the Great Basin) was deepened, the Great Stone Dock was rebuilt and a new dry dock (known today as No 4 dock) was built alongside it over a five-year period from 1767. During 1771-76 the former Upper Wet Dock was reconfigured to serve as a reservoir into which water from the dry docks could be drained by way of culverts (enabling ships to be dry docked much more speedily). From 1789 work was begun on replacing the old wooden South Dock with a modern stone dry dock (known today as No 1 dock, it currently accommodates the museum ship HMS "M33").