The Binnian Tunnel (2.5 miles long) was built between 1947 and 1950/51 underneath Slieve Binnian, to transport water from the Annalong Valley to the Silent Valley. A workforce of 150 was involved in two tunnelling teams which started from opposite ends and met in the middle nearly 800m under the roof of the mountain. Having been carefully measuring their positions the whole time, they found they were only two inches off course when the two tunnels met.