Complex sentence: At the time of its construction, underwater tunnels were a well-established part of civil engineering, but no long vehicular tunnel had been built: the technical hurdle was the ventilation required to evacuate the carbon monoxide emissions, which would otherwise asphyxiate the drivers.
Split into simpler sentences: At the time of its construction, underwater tunnels were a well-established part of civil engineering, but no long vehicular tunnel had been built. The technical hurdle was the ventilation required to evacuate the carbon monoxide emissions, which would otherwise asphyxiate the drivers.