From the NY Times:
On Tuesday, manhole fires in Brooklyn, Queens and the Bronx prompted evacuations of nearby homes.
All these incidents — only one of which involved an injury, minor — are part of an annual cycle as predictable as crocuses in March and mosquitoes in June: the surge in manhole fires as salty slush seeps down into the city’s maze of subterranean utility closets. There have been at least 19 such fires and explosions since New Year’s Day, according to Consolidated Edison and fire officials.
Salt laid down on wintry streets corrodes cables and makes them more susceptible to catching fire, said a Con Edison spokesman, Chris Olert. A smoldering cable can also release noxious gasses that can build to such intense pressure that the manhole cover pops off. Con Edison has redesigned many of its manhole covers to increase ventilation and to decrease the chance of such a frightening occurrence.
The fires happen more often in Manhattan and other areas where electrical wiring runs underground. In Staten Island and parts of Queens, many of the wires run aboveground, making them impervious to manhole fires but more vulnerable to high winds and storms.
Anyone who sustains an injury or has property damaged in a manhole fire can file a claim with Con Edison.