Between 1947 and 1951 over 150 men drove a tunnel underneath Slieve Binnian. The tunnel was built to carry water from the Annalong valley to the Silent Valley Dam, which had been completed in 1933, 14 years earlier. This was to further supply the growing demands of Belfast's water supply. Two work squads began at each end of the tunnel, and met half way almost 800 metres under the mountain. The technology in the day was lacking in every standard, and was lit by candlelight. When the two squads met, they were mere inches off. The tunnel measures square - and long. Its entry is at Dunnywater and its exit is on the roadside, a short distance from the visitor's centre. The Binnian Tunnel was officially opened on Thursday, 28 August 1952.