I know somebody requested proof of molten steel under the rubble at ground zero. Here are some accounts for you:
Molten Metal: Flowing and in Pools
There are several published observations of molten metal in the basements of all three buildings, WTC 1, 2 ("Twin Towers") and 7. For example, Dr. Keith Eaton toured Ground Zero and stated in The Structural Engineer,
'They showed us many fascinating slides' [Eaton] continued, 'ranging from molten metal which was still red hot weeks after the event, to 4-inch thick steel plates sheared and bent in the disaster'. (Structural Engineer, September 3, 2002, p. 6; emphasis added.)
The observation of molten metal at Ground Zero was emphasized publicly by Leslie Robertson, the structural engineer responsible for the design of the World Trade Center Towers, who reported that "As of 21 days after the attack, the fires were still burning and molten steel was still running." (Williams, 2001, p. 3; emphasis added.)
Sarah Atlas was part of New Jersey's Task Force One Urban Search and Rescue and was one of the first on the scene at Ground Zero with her canine partner Anna. She reported in Penn Arts and Sciences, summer 2002,
'Nobody's going to be alive.' Fires burned and molten steel flowed in the pile of ruins still settling beneath her feet. (Penn, 2002; emphasis added.)
Notice that the molten metal was flowing down in the rubble pile early on; so it is not the case that the molten metal pools formed due to subterranean fires after the collapses. Dr. Allison Geyh was one of a team of public health investigators from Johns Hopkins who visited the WTC site after 9-11. She reported in the Late Fall 2001 issue of Magazine of Johns Hopkins Public Health, "In some pockets now being uncovered they are finding molten steel."
A video clip provides eye-witness evidence regarding this metal at ground zero:
http://www.youtube.com/watch?v=4p6UuGE0plk
The observer notes that the observed surface of this metal is still reddish-orange some six weeks after 9-11. This implies a large quantity of a metal with fairly low heat conductivity and a relatively large heat capacity (e.g., iron is more likely than aluminum) even in an underground location. Like magma in a volcanic cone, such metal might remain hot and molten for a long time -- once the metal is sufficiently hot to melt in large quantities and then kept in a fairly-well insulated underground location. Moreover, as hypothesized below, thermite reactions may have resulted in substantial quantities (observed in pools) of molten iron at very high temperatures -- initially above 2,000 °C (3,632 °F). At these temperatures, aluminum materials from the buildings should continue to undergo exothermic oxidation reactions with materials also entrained in the molten metal pools including metal oxides, which will then keep the pools molten and even growing for weeks despite radiative and conductive losses.
THERE's some evidence for you.