Dodecahedral hydrocarbon that took nearly 20 years to synthesise properly. Making parts of the structure wasn't too hard (apparently - obviously I didn't do it) but closure was tricky. Although the final product is very stable due to its symmetry, the final bridging was tough. One of the solutions involved adding cyclopentadiene to a flower shaped base - like putting the lid onto a box. Another involved a coiled strip of cyclopentane (or five membered rings, anyway) 'glued' together at the edges. Pointless, but pretty.