Holland Tunnel Ventilation Shaft 2

This massive structure is one of four ventilation towers designed to keep the air breathable deep beneath the Hudson River. When the Holland Tunnel opened in 1927, it became the first underwater vehicular tunnel in the world to feature a mechanical ventilation system. Engineers had to solve the critical problem of carbon monoxide buildup from internal combustion engines, which threatened to turn the tunnel into a deadly gas chamber. They turned to the University of Illinois and the United States Bureau of Mines, which conducted physiological tests in an old Pennsylvania coal mine to determine how much exhaust human beings could safely tolerate. The result was a transverse-flow ventilation design that remains the global model for long tunnels today.