What could cause MacBook battery cells to swell up? I have a problem with my 3 year old MacBook. The battery has "swollen up" (I could post a picture if that is of help).

One side of the battery is much bigger than the other. Since it no longer works, I opened it. Inside it is 6 cells covered in metal envelopes. 2 of the 6 cells are about twice as big as they ought to be.

What do Apple's batteries contain? What could be the reason for battery cells to swell up?
Apple battery packs contain a set of 6 lithium Ion (Li-Ion) cells (the metal covered envelopes), connectivity circuitry, and a small logic board.

The main cause for battery cell swelling is what is known as thermal runaway.

If a cell reaches a certain temperature then a chemical in the cell (usually Cobalt Oxide for Lithium Ion batteries) starts an unintended reaction which is exothermic in nature (i.e. it creates heat).  More heat raises the cell temparature further which causes more of the same chemical reaction

This creates a runaway thermal overload which can end in one of two ways, the cell and therfore the battery pack, catches fire or the reaction fizzles out with the cell in a swollen state.