In 2004 a French film maker,  Etienne Sauret,  produced a documentary titled The First 24 Hours, in which rescue workers recount, among other things, 'rivers of molten steel' under the rubble of the collapse WTC towers. Official reports,  such as this one,  from the American Society of Safety Engineers, tell a similar story:

The debris pile at Ground Zero was always tremendously hot. Thermal measurements taken by helicopter each day showed underground temperatures ranging from 400 deg F to more than 2,800 deg F. The surface was so hot that standing too long in one spot softened (and even melted) the soles of our safety shoes. Steel toes would often heat up and become intolerable ... The underground fire burned for exactly 100 days and was finally declared 'extinguished' on Dec. 19, 2001.

An assorted selection of referenced quotes follows (source) :

An employee of New Jersey's Task Force One Urban Search and Rescue witnessed "Fires burn[ing and molten steel flow[ing] in the pile of ruins still settling beneath her feet."

The head of a team of scientists studying the potential health effects of 9/11, reported, "Fires are still actively burning and the smoke is very intense. In some pockets now being uncovered, they are finding molten steel."

A public health advisor who arrived at Ground Zero on September 12, said that "feeling the heat" and "seeing the molten steel" there reminded him of a volcano.

New York firefighters recalled in a documentary film, "heat so intense they encountered rivers of molten steel."

According to a worker involved with the organizing of demolition, excavation and debris removal operations at ground zero, "Underground it was still so hot that molten metal dripped down the sides of the wall from Building 6."

An expert stated about World Trade Center building 7, "A combination of an uncontrolled fire and the structural damage might have been able to bring the building down, some engineers said. But that would not explain steel members in the debris pile that appear to have been PARTLY EVAPORATED in extraordinarily high temperatures" (pay-per-view). Note that evaporation means conversion from a liquid to a gas; so the steel beams in building 7 were subjected to temperatures high enough to melt and evaporate them.

A reporter with rare access to the debris at ground zero "descended deep below street level to areas where underground fires still burned and steel flowed in molten streams."

The same journalist also refers to "the streams of molten metal that leaked from the hot cores and flowed down broken walls inside the foundation hole." (pages 31-32)

An engineer stated in the September 3, 2002 issue of The Structural Engineer, "They showed us many fascinating slides ranging from molten metal, which was still red hot weeks after the event."

An Occupational Safety and Health Administration Officer at the Trade Center reported a fire truck 10 feet below the ground that was still burning two weeks after the Tower collapsed, "its metal so hot that it looked like a vat of molten steel."

The structural engineer responsible for the design of the WTC, described fires still burning and molten steel still running 21 days after the attacks.

According to a member of New York Air National Guard's 109th Air Wing, who was at Ground Zero from September 22 to October 6, "One fireman told us that there was still molten steel at the heart of the towers' remains. Firemen sprayed water to cool the debris down but the heat remained intense enough at the surface to melt their boots."

A fireman stated that there were "oven" like conditions at the trade centers six weeks after 9/11.

Firemen and hazardous materials experts also stated that, six weeks after 9/11, "There are pieces of steel being pulled out [from as far as six stories underground] that are still cherry red" and "the blaze is so 'far beyond a normal fire' that it is nearly impossible to draw conclusions about it based on other fires." (pay-per-view)

A NY Department of Sanitation spokeswoman said "for about two and a half months after the attacks, in addition to its regular duties, NYDS played a major role in debris removal - everything from molten steel beams to human remains...."

As late as five months after the attacks, in February 2002, firefighter Joe O'Toole saw a steel beam being lifted from deep underground at Ground Zero, which, he says, "was dripping from the molten steel."

Indeed, the trade center fire was "the longest-burning structural fire in history", even though it rained heavily on September 14, 2001 and again on September 21, 2001, and the fires were sprayed with high tech fire-retardands, and "firetrucks [sprayed] a nearly constant jet of water on" ground zero."

Indeed, "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."

Could the fires that were burning in the towers at the time of collapse have created this underground inferno, or was some other process responsible?

A combination of an uncontrolled fire and the structural damage might have been able to bring the building down, some engineers said. But that would not explain steel members in the debris pile that appear to have been PARTLY EVAPORATED in extraordinarily high temperatures (The New York Times).

Appendix C. of the FEMA Report on 9/11 contains an article titled Limited Metallurical Examination. The article describes the unusual thinning of steel due to an attack of the steel by grain boundary penetration of sulfur forming sulfides that contain both iron and copper (C-5).  This suprisingly brief article ends with the following tantalising statement:

The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified. The rate of corrosion is also unknown. It is possible that this is the result of long-term heating in the ground following the collapse of the buildings. It is also possible that the phenomenon started prior to collapse and accelerated the weakening of the steel structure. A detailed study into the mechanisms of this phenomenon is needed to determine what risk, if any, is presented to existing steel structures exposed to severe and long-burning fires (C-13).

Materials science professors  Ronald R. Biederman and Richard D. Sisson Jr confirmed that the WTC steel had been subjected to a eutectic reaction, causing intergranular melting capable of turning a solid steel girder into Swiss cheese:

A one-inch column has been reduced to half-inch thickness. Its edges--which are curled like a paper scroll--have been thinned to almost razor sharpness. Gaping holes--some larger than a silver dollar--let light shine through a formerly solid steel flange. This Swiss cheese appearance shocked all of the fire-wise professors, who expected to see distortion and bending--but not holes.

The possibility, noted in Appendix C of the FEMA report,  that the phenomenon 'started prior to collapse and accelerated the weakening of the structure' is indeed worrisome. It is odd that the subsquent $20 million NIST study, whose object purportedly was to explain the collapses, declined to follow up on the preliminary findings of Appendix C or the research of Biederman and Sisson Jr.

Perhaps there is a reason why no one bothered to follow up on research which would help us to understand why the WTC, a building designed to withstand the impact of a 707, shattered. Or, since everyone knows that truth is stranger than fiction, the experts may have decided to skip this question.