Board of Water Supply. City Water Tunnel No. 1. A Mattson compressed air lock in use during the sinking of a City Water Tunnel No. 1 shaft in the East Village. Pneumatic caissons, sealed off and filled with compressed air, were constructed to pass through up to 100 feet of water-bearing material overlying the bedrock. The air lock enabled safe passage for workers between areas of differing air pressure. October 20, 1911.