New York's underground rail system is in danger of being flooded because of last week's terrorist attack.
Engineers are drawing up emergency plans to prevent water from the Hudson pouring into the seven-floor basement of the World Trade Centre. They fear water will get into the subway system and fill train tunnels across the city.
Dan Hahn, of Mueser Rutledge Consulting Engineers, which is working on the disaster site, said: "It could flood a lot of the underground system of New York."
News of the flood danger was disclosed on a website run by New Scientist magazine.
The basement beneath the demolished twin towers housed shops and parking, with a subway station at the bottom. Two train tunnels from the station head west out of the basement, beneath the Hudson, where they join other tunnels.
During building, the walls of the basement were secured against the pressure of surrounding soil and water by anchoring them to the bedrock with steel cables. When completed, the basement floors braced against the walls took the strain, and the cables were cut.
With the basement floors now destroyed or damaged, engineers believe only debris is preventing the walls from collapsing in on themselves. If that happened it would let mud and water pour in.
The engineers plan to get into the tunnels – not expected to be possible for weeks – and block the tunnels with huge concrete plugs.Reuse content