At once reptilian and avian, dinosaurs enable us to imagine a world far beyond the usual boundaries of time, culture and physiology. We have envisioned them in diverse and contradictory ways, reflecting, in part, our changing conceptions of ourselves. Their discovery, around the start of the nineteenth century, was intimately tied to our awareness of geological time; their vast size and power called to mind railroads, battleships and factories, making dinosaurs, paradoxically, emblems of modernity. At the same time, their world was nature at its most pristine, and they simultaneously symbolized childhood innocence and wonder. Dinosaurs provided a sort of code that has enabled people to speak indirectly about the possibility of our own extinction. Not unlike humans today, dinosaurs seem both powerful - almost godly - and helpless in the face of cosmic forces even mightier than themselves.