Officials Harden New York-Area Tunnels after Threats

Electronic monitoring, response systems considered, but most subway tunnels above sea level

PATH tunnels partially flooded during a violent 1992 storm, trapping one train. Passengers escaped by fleeing through waist-deep water.

After the attack on the World Trade Center in 2001, authorities were concerned that the collapse had weakened a huge underground wall that keeps Hudson River water out of the twin towers' basement, which also contains a PATH station. The wall held.

A 1995 study of possible subway flooding patterns said the parts of the system most susceptible lay below 26th Street. In much of the rest of Manhattan, subway stations are above sea level.

In a frightening scenario, a fired MTA deputy director warned last year that a sudden flood caused by a bomb in an East River tunnel - one not protected by bedrock - might kill thousands of people.

The MTA's Schiliro called that prediction "foolhardy" and speculative, but said it didn't take away from the need to armor the tunnels against possible assault.

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