New blockships were sunk, booms and mines were placed over the main entrances, coast defence and anti-aircraft batteries were installed at crucial points, and Winston Churchill ordered the construction of a series of causeways to block the eastern approaches to Scapa Flow; they were built by Italian prisoners of war held in Orkney, who also built the Italian Chapel. These "Churchill Barriers" now provide road access from Mainland to Burray and South Ronaldsay, but block maritime traffic. An air base, RAF Grimsetter (which later became HMS "Robin"), was built and commissioned in 1940.