At the time of its construction, the Brooklyn-Battery Tunnel System was the longest continuous underwater tunnel in the world; today, it remains the longest in North America. The tunnel was designed by engineer Ole Singstad, Chief Engineer for New York City, who utilized an innovative system by which a trench is burrowed into the river bed with cast-iron rings being the principal structural component of the tunnel. The road bed is then laid into the 31-foot diameter tunnel. Above ground reinforced concrete ventilation towers, designed with a simple, geometric Streamline Moderne motif, are capable of exchanging the entire volume of air in the tunnels every 90 seconds.