When authorities announced last week that they had foiled a plot to bomb commuter train tunnels under the Hudson River, they alleged that the suspects hoped such attacks would trigger even broader destruction: a flood.
But could that really happen?
For years, security officials have worried that a powerful bomb detonated in one of New York's underwater tunnels could send a torrent of water cascading through the city's labyrinth of subterranean tubes, flooding the subways and drowning commuters.
Experts are skeptical that a bomb small enough to be hidden in a bag or backpack could cause a breach in river tunnels drilled through bedrock, as many of the city's subterranean tubes are. Train operators, however, have been spending millions to "harden" key tunnels to protect them in case of a bomb attack.
"It's obviously a very serious concern of ours," said Lewis Schiliro, a former FBI agent who is now director of interagency preparedness for the Metropolitan Transportation Authority, which runs the subway system.
Authorities declined to discuss specifics of their efforts, but one element announced in the spring was a $17.1 million contract to pile extra earth and concrete atop underwater tunnels to lessen the likelihood of a leak.
In a transit system with 14 underwater tubes leading to Manhattan alone, and river portals nearly a century old, the total cost of improvements could be significantly higher.
"When you think about hardening a tunnel, you can't just go into Home Depot and buy a tunnel-hardening kit," Schiliro said. He declined to give a specific cost.
The MTA has also improved "intrusion detection" in critical tunnels, Schiliro said. Police are posted around the clock at the entrances to some underwater tubes, and other areas are monitored electronically, he said.
Amtrak, which has tunnels across both the Hudson and East rivers, is part way through a $472 million project to make the tubes less likely to turn into death traps during an attack or major accident.
The improvements, begun in 2002, include powerful fans that can ventilate smoke, more water sources in the tunnels for firefighters and updates to the flood gates designed to seal off the tunnels in the event of a breach.
Amtrak spokesman Cliff Black said the work should be complete by 2009.
The complicated geography of Manhattan's underground infrastructure makes it difficult to say how vulnerable parts of it are to a flooding attack. There are 21 train and car tunnels across the Hudson, East and Harlem rivers. Only those that terminate below sea level have the capability of spreading flooding into Manhattan subways.
Two law enforcement officials said Thursday that the suspects in the flooding plot had hoped to unleash the Hudson River on the city, not by their direct attacks on the tunnels but by destroying an underground wall that keeps water from entering the World Trade Center site.
After the attack on the Trade Center in 2001, authorities were concerned that the collapse had weakened the slurry wall and that a breach could fill the ground zero pit with water and flood PATH rapid transit tubes leading back toward New Jersey.
The wall held and is now being reinforced. The law enforcement officials, who spoke on condition of anonymity because the flooding scheme was still under investigation, did not know how the attackers planned to destroy the wall.
Allan McDuffie, an Army Corps of Engineers expert who studied how New York's transit system might be damaged by hurricane flooding, said it would take "quite a charge" to spring a leak in the PATH system, the commuter line to New Jersey that authorities allege was targeted.
The PATH is buried under bedrock, he said, which means a breach would require the kind of detonation more likely to be caused by truckloads of explosives than a backpack bomb.
"I think it would be quite a task to get an explosion big enough to breach a PATH tunnel, but, if you assume it could, then the PATH system is connected into subway tunnels, so it is possible it would cause flooding," McDuffie said.
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.