Experimental evidence that fullerenes are built by the breakdown of larger precursors and not constructed atom by atom in nature has emerged from work by Harry Dorn of Virginia Tech and colleagues. Although several processes for making fullerenes are well documented, there are two competing theories about the mechanisms at work at the molecular level. The first and oldest is the bottom-up theory, which says these carbon cages are built atom-by-atom. The second, more recent theory, takes a top-down approach, suggesting instead that fullerenes are formed when much larger structures break into constituent parts. Dorn's study of a particular metallofullerene consisting of 84 carbon atoms with two additional carbon atoms and two yttrium atoms trapped inside revealed that this structure could collapse to form almost every other fullerene, as revealed by X-ray analysis.