Proponents of the micro-nuke theory of controlled demolition have argued that the molten metal found weeks after 9/11 cannot be explained by thermite, since fire requires oxygen. They say the red-hot metal underground proves that nuclear devices were used on 9/11.

It was pretty easy to find the error in the above argument. Thermite does not require atmospheric oxygen in order to burn, so a thermite fire could continue burning underground.

Thermite is a mixture of iron oxide and aluminum.
THE OXYGEN FOR THE REACTION IS PROVIDED BY THE IRON OXIDE.

Here is the equation:

FE2 O3 + 2AL ---> AL2 O3 + 2 FE
Thermite = Iron Oxide + Aluminium ---> Aluminium Oxide + Iron + Heat
(from http://en.wikipedia.org/wiki/Thermite)

The same amount of oxygen goes into and comes out of the reaction; it just changes over from iron oxide to aluminium oxide.

This makes sense - if the reaction depended on oxygen from the air, it wouldn't burn so hot.
Air is only about 20% oxygen and is also very diluted being a gas, compared to a solid like iron oxide - that's why some rockets use solid fuel.

As to whether a thermite fire could burn underground for weeks, I've never tried, of course! However, applying the same potted kitchen-stove physics that told me years ago, Steel doesn't weaken from a hydrocarbon fire in open air, I conclude that an underground thermite fire might keep going for some time.

In my very same kitchen where I discovered that steel pots do not melt on the stove (Eureka!), I have found that a pot of stew will stay piping hot all night if you turn off the heat and put a towel around it, just as a blanket keeps you warm all night.

So rather than suffocating a thermite fire, the surface layers of debris would actually keep the heat in. Granted, my pot under a towel has only been tested for one night, but it is quite a small hot body with a very thin layer of insulation. The molten pools were much more massive with much thicker insulating material over them.

Moreover, the flames that were seen to flare when bulldozers pulled up debris several weeks later could have come from the stirring action of the earth-mover, bringing unburned thermite into contact with hot surfaces and igniting it. Remember the old campground wisdom - beware of hot embers under an "extinguished" fire.

Conclusion: other evidence than the molten pools of metal is needed to solve the interesting question, Were there micro-nukes in the WTC?