Now, in a somewhat unrelated note, this might tie in with one of my "wacky" theories about ancient civilizations having computers; what if we eventually make all computers completely (& relatively quickly) bio-degradable, then, in a couple of hundred years after a city is abandoned, there may not be any trace of that city having computers left for future civilizations to discover. This kind of thing could have happened in the past. I think that it is possible that flight, nuclear weapons, computers, etc. could have been invented & used at some earlier points in the 200,000 year long human past; or that as our modern age discovers them, this may not be the first time that those high tech gadgets have made an appearance on Earth. Perhaps that's a theory best left to a sci-fi movie script :) But, the drive to create both high-tech & eco-friendly products does make me wonder.