An estimated 350,000 riders were stranded on New York subways and thousands more were stuck on Metro-North and Long Island Rail Road trains after a massive power outage left much of the northeast in the dark. The blackout, which occurred at 4:11 pm on Thursday, was caused by a power grid failure. Power was restored by Friday morning to sections of all five boroughs. Transit officials, said the restoration of power to about half of New York wasn't enough to bring the subways back, and said they likely would remain down through the evening rush hour. On the first day of the black out, subway passengers were trapped in pitch-black, sweltering tunnels, forced to clamber across the tracks and through trapdoors to safety and daylight, reported the Daily News. Transit Authority spokesman Paul Fleuranges said subway workers did a good job of keeping riders calm, sweeping the tubes and leading panicky commuters to the streets, according to the paper. By nightfall, the 720-mile system was still out of service, with officials saying it would take six to eight hours after all power was restored to get the trains running again. The cause of the blackout, which knocked out service in parts of eight states and Canada in just nine seconds, is still a mystery.