Analysis / Ragnarok-Proofing
Metal, no matter how well protected, will eventually succumb to the elements and corrode. After about 75 years, cars and other large machines will turn into almost unrecognizable piles of rust while the fuel, oil and other lubricants inside them will go bad long before that. (It is common practice to drain or put fuel stabilizer chemicals in the gasoline tank of equipment not being used for a few months, like lawn cutting equipment and boats during late fall, winter and early spring; snowmobiles during early spring through late fall, etc.) The same fate awaits the rubber in the tires, hoses and accessory belts, which will inevitably crumble from dry rot. Long before any of that happens, the car will have become an immobile lump as batteries lose their charge, tires slowly leak air, and the brakes rust up and seize from disuse, conditions known as "lot rot". Large scale structures fare no better. In many climates, wooden frame buildings will last about 50 years before falling apart thanks to termites and rotting. Large bridges will collapse after only a century, and most skyscrapers will collapse around the 200-300 year mark. After 500 years, nearly all concrete structures still standing will crumble as their steel reinforcements corrode. See The History Channel's Life After People for more information. And this is all assuming that a natural disaster like a tornado, earthquake, or hurricane doesn't destroy it all first (how much of Florida would survive 10 years if people weren't around to board everything up each summer?). It also ignores the likelihood that whatever arises after the fall of society would knock it down/scavenge it themselves instead of just waiting for nature to do the job. Modern technology isn't immune either. All but the simplest electronics will fail after decades of being unused. Electrolytic capacitors dry out (or succumb to the capacitor plague), batteries self-discharge and leak, flash memory very slowly fades away, and the chassis and contacts rust and corrode. Lead free solder grows tin-whiskers, creating short circuits; hard discs rot or degrade, and the skin of optical media such as Blu-Rays and DVDs corrodes, rendering the disc illegible (aka "CD rot"). After a thousand years of no human activity, the Earth would look much like it was before humans, and few obvious traces of civilization would be left. Some plastic types, if buried underground (away from UV radiation) would keep for a long time until something figured out how to properly eat them; anything made out of bronze is expected to last for millions of years (so cast your memoirs with it); major cities, being massive conglomerations of artificial rock on the scale of a coral reef or lava flow, would leave traces in the geological record discernible for several hundred million years; depleted uranium would remain detectably depleted for billions of years — but none of this would be visible to a casual observer, or even a medieval society, and little of it would be immediately recognizable to future visitors.