effect but due to an administrative error it was not formally registered, leaving the land in the legal ownership of the Navy, which successfully sought its return in 1866. Sandstone fortifications, built on the island during the 1820s to protect Sydney from a much-feared Russian attack, also survive. Garden Island also boasts what is claimed to be Australia's first lawn tennis court. Built in 1880, it is still in use, although the lawn was replaced in 1960. Captain Cook Graving Dock Prior to World War II, the nearest sizable naval graving dock was at Singapore Naval Base; an round trip for a potentially damaged warship. Consequently, in 1938, the Australian cabinet approved the idea of building a large naval graving dock. The cost of construction was predicted at around A£3 million. A far cheaper alternative, a second-hand floating drydock being sold by Southern Railway was considered early on. Despite the A£175,000 cost, the acquisition was opposed by Admiral Ragnar Colvin, as it would be expensive and difficult to maintain, would be unable to accommodate the draught of ships being acquired for the RAN, and would be incredibly risky to tow from England to Australia. Three sites were considered (one in Adelaide, and two in Sydney Harbour: Potts Point or near the Sydney Harbour Bridge), with Potts Point chosen as the cheapest location. The dock itself was built by the reclamation of of land, connecting Garden Island to the mainland. By September 1944, work had been completed to the stage where initial flooding tests could occur. On 2 March 1945, the British aircraft carrier underwent an emergency docking: although the drydock was not due to open for another three weeks, the advanced state of building made the docking possible. The Captain Cook Graving Dock was formally opened by Prince Henry, Duke of Gloucester in his role as the Governor-General of Australia, with the ribbon cutting performed by the bow of the frigate . During the first year of operations, the drydock received the British battleship ; the 45,360-ton displacement warship is the largest vessel to use the dock. The dock is long, with a width of . The dock is deep at spring tide. When filled, the dock has a capacity of . The dock can be drained in a four-hour period through the use of three centrifugal pumps (each with a discharge capacity around per minute). Hammerhead crane The hammerhead