Even as MacGuffins go, the unobtainium in the hit movie Avatar was, to be charitable, a pretty thin plot device. As movie fans know, a MacGuffin is a Hitchcock-style catalyst that drives the plot of a film forward without really adding anything, such as "government secrets" in North by Northwest or the stolen 40 grand in Psycho.
MacGuffins are typically ambiguous or even unidentified. What was in the briefcase in Pulp Fiction? What were those "letters of transit" in Casablanca, or the "top secret plans" in The 39 Steps? We'll never know, and if we did, the yarn in question might not have been so compelling.
Equally vague is Avatar's unobtainium. It is, apparently, a rare and costly mineral ("20 million a kilo") that is supposed to make the military human presence on the planet Pandora somewhat plausible--if you didn't see the flick, think Iraq and oil. You never get to see the stuff, and its use -- something about matter-antimatter reactors (of course!) -- is left fuzzy beyond measure: It's crucial enough for Earth to go to war with the peaceful Na'vi inhabitants of the planet, but not sufficiently important to be fleshed out in any meaningful way. All rather unsatisfying. (Not that much of the movie isn't quite wonderful -- I could have happily skipped the plot and spent a couple of hours just wandering around the refulgent neon-blue three-dimensional wonderland of Pandora.)
The concept of unobtainium has been around for decades, in the sense of a substance that fulfills some particularly tricky task -- like the gravity-insulating material 'cavorite' in H.G. Wells' First Men on the Moon, or 'scrith' in Larry Niven's Ringworld -- but which, in the everyday world, has the singularly unfortunate quality of non-existence.
Or maybe it does exist but is really hard to obtain. Lockheed engineers referred to titanium as unobtainium when developing the SR-71 Blackbird spy plane. Titanium was needed for the skin of the plane because of its characteristic resistance to high temperatures, and back then the Soviet Union had cornered the market for the metal. (Since that time, a process has been invented to refine metallic titanium from Florida's deposits of titanium dioxide sand.)
These days, unobtainium might be the magic battery ingredient that will allow us to store a week's worth of electrical energy lightly and efficiently in trucks, buses, trains and automobiles. I bet there's a whole bunch of it on Pandora.
Barry Evans (firstname.lastname@example.org) wonders if "lasting happiness" is the unobtainium of the emotions