Below the streets of New York City, a network of pipes, cables and tunnels up to 200 feet deep transports power, gas, water, Internet traffic, trains, sewage and more. When Tropical Storm Irene hit the city Sunday, this underground network was largely protected from major damage.
On the island of Manhattan, only a handful of the 1.6 million residents lost power. And roughly 50,000 households in the city's outer boroughs of the Bronx, Brooklyn, Queens and Staten Island lost power.
Elsewhere, Irene left millions of East Coast residents without power, as winds knocked trees into above-ground power lines.
The city's buried infrastructure is safe from wind, but it is vulnerable to flooding. Some experts say the city simply got lucky that the flooding wasn't more severe. Its vast subway system, with 735 stations and 2,000 miles of track, is especially at risk. That's why transportation officials preemptively shut it down and it remained closed Sunday evening.
Forecasters feared Irene would deliver a surge twice as high as the 4 feet extra of water that washed over parts of the southern tip of Manhattan.
"If the surge had been three feet higher, there could have been huge damage," said Martin Bowman, professor of oceanography at the Marine Sciences Research Center of the State University of New York at Stony Brook. Bowman heads a group at Stony Brook that studies storm surges.
A large storm surge, like one that hit nearly 200 years ago, could flood crucial parts of the subway system and cripple a transportation network that delivers 5 million passengers every weekday. Power plants and airports near the city's extensive shoreline could be knocked out. Wall Street could be flooded.
Directly beneath Manhattan's streets lie 21,000 miles of power lines used by Consolidated Edison to deliver electricity. The lines and equipment are accessed through 60,000 manholes and service boxes. Most of the equipment underground is sealed and not vulnerable to flooding, said Arnold Wong, a project manager for high-voltage transmission at ConEd.
But ConEd warned that it might need to cut off power to 17,000 customers in Lower Manhattan, home to the financial district, because saltwater would have been more damaging to equipment if power were still coursing through wires.
Hair-thin optical fibers run underground near the electrical wires. They carry the city's phone calls and Internet traffic and data crucial to the financial markets. Optical fiber isn't as sensitive to flooding as power lines, though it does rely on power to operate.
In the 30 feet of ground below the power and telecommunications lines lies a mesh of steam, water and gas lines. ConEd shut down steam service to some customers before the storm, but no other services were disrupted and little damage appears to have been done.
Far below those lines _ and below the famous subway system _ the city's sewer system runs. It is an antiquated system that carries sewage and storm runoff together to water treatment plants. Even moderate storms overwhelm the system and force officials to release untreated sewage into the city's waterways. In preparation for Irene, officials began releasing sewage days ahead of the storm.
By far the biggest concern before Irene was that a storm surge would send seawater cascading into subway entrances, flooding tracks and ruining control equipment.
Subway tunnels can be as deep as 180 feet underground or just one story below the street, but they generally sit below the power, cable, water and gas lines, and above the sewer line.
Water is a constant problem. On a typical day, 13 million to 15 million gallons of water is pumped out of the subway system. But that is only a tiny fraction of what could be brought by a major storm surge. If a surge of water began to pour into a subway entrance or through grates that line the sidewalk, the pumps would be overwhelmed.
That would destroy the subway's electrical and hydraulic systems, and the water damage would be compounded by the corrosive nature of sea salt. Water would also flow downhill into the long, deep tunnels that pass under the East River between Manhattan and Brooklyn and Queens, further crippling the system. If water rises to a track's third rail, where electric power is delivered to the train, trains cannot run.
Lower Manhattan contains a high concentration of subway stations that serve nearly all the city's lines. Throughout the system, Bowman counts 25 subway stations whose entrance stairways are about 15 feet or so above sea level, while most are on higher ground.
In the end, Irene did not deliver as strong a blow as feared. Transportation officials declined to predict Sunday when the subway system would be back up and running. Inspectors are checking the tracks and pumper trains have been sent to places where flooding from rainwater occurred. After that is finished, trains without riders must be sent through the system to test it.
Still, Irene offered a troubling glimpse of what might be possible if a future storm brings more intense flooding. While a storm like Irene is a rare event in New York, it is not unheard of.
In 1992, a winter storm drove an 8-foot surge that flooded the entrance of the underground commuter train station in low-lying Hoboken, N.J., just across the Hudson River from Manhattan. Half a mile of track was inundated, and the train system was out of service for ten days.
An 1821 hurricane drove an even higher surge. The exact extent is not known, but the water rose 13 feet in one hour at Manhattan's southern tip, according to the city government's website.
Hurricane expert Jeff Masters cites a National Oceanic and Atmospheric Administration model predicting that a Category 2 hurricane could drive a 15- to 20-foot surge into New York. That would flood JFK Airport and Manhattan as far north as Canal Street.
Bowman says New York should build flood barriers to protect the city, much like London has had since building the Thames Barrier in 1982.
Bowman believes threes such barriers are necessary to protect New York: one across the upper East River, to shield against surges funneled through Long Island Sound; one across the narrow waters between Staten Island and New Jersey; and most dauntingly, one spanning from New York's Rockaway beach community across five miles of water to New Jersey's Sandy Hook.
"It's not as big of an engineering problem as you might think," Bowman said, because apart from shipping channels, the water of the Lower New York Bay is only 20 feet deep. He puts the cost of the entire project at $5 billion to $10 billion.