How Did the WTC Fires Burn for Months?

The fires in the wreckage of the World Trade Center buildings burned for months despite the best efforts of the weather and firefighters to extinguish them...

Rain Hampers WTC Recovery Efforts - September 14, 2001

Rescuers racing against the clock in a desperate attempt to find survivors in the World Trade Center's rubble faced a new obstacle Friday: rain. Three days after hijacked passenger jets plowed into the twin 110-story skyscrapers, thunder and lightning brought a torrent of rain to the mammoth heap of ash and twisted wreckage. But sodden rescue workers kept at it during downpours that began around 1 a.m. [Court TV]

Rain Hampers Rescue Effort - September 21, 2001

[H]undreds of rescue workers in yellow slickers continue to dig through the wreckage. They're working amid some heavy rain and lightning. Earlier this morning, a 20-minute lightning storm stopped much of their rescue efforts and sent them scurrying for shelter. The rain is also making it hard for crews to keep their footing. [WNBC]

WTC Fires All But Defeated - December 19, 2001

Firefighters have extinguished almost all but the last remnants of underground fires that have burned at the World Trade Center site for more than three months since the Sept. 11 terrorist attack. The fires that began with the Sept. 11 attacks had been strong enough that firetrucks had to spray a nearly constant jet of water on them. At times, the flames slowed the work of clearing the site. "You couldn't even begin to imagine how much water was pumped in there," said Tom Manley of the Uniformed Firefighters Association, the largest fire department union. "It was like you were creating a giant lake." [CBS News]

Thermite induced collapses would explain the above:

Thermite contains its own supply of oxygen, and does not require any external source such as air. Consequently, it cannot be smothered and may ignite in any environment, given sufficient initial heat. It will burn just as well while underwater, for example, and cannot even be extinguished with water, as water sprayed on a thermite reaction will instantly be boiled into steam. [Answers.com]