from above isn't nearly enough. manhattan, home to a million and a half people, is an island. much of its sensitive infrastructure is on the water, including possible terrorist targets, like the giant ventilation shaft that circulates air in the tunnel. the united nations, the brooklyn bridge and a power plant. >> it gets difficult to think how we're going to protect these infrastructures. >> the biggest problem could be the monorails, all the bridges and piers. each one needs to be physically inspected by scuba teams. >> we do it every single day. every day of the year we're down there looking for any kind of irregularity. >> the waters of new york are polluted and cold. the divers wear vulcanized rubber suits and full face masks. >> you don't want the water to