After Man
What would the world end up looking like if we all vanished tomorrow? Flipside 28 discusses likely scenarios the last human alive, like Will Smith in blockbuster I Am Legend, might be faced with. But what about in the really long term...?
 After thousands of years, would visiting aliens be able to tell we had ever existed?
Science writer Alan Weisman performed this detailed thought experiment for his book The World Without Us. He concluded that within 500 years the majority of urban areas would have been covered by forests, eventually scraped clean of human-built remains by expanding glaciers during Ice Ages to come.
The aliens might only get to find out only what we looked like if they stumbled across newspapers preserved in landfills – which don’t degrade if separated from air and water – or else make it to Mount Rushmore in South Dakota, where the granite faces of five US presidents should remain vaguely recognisable for the next 7.2 million years.
 Humanity’s single most lasting legacy will be geological: more than one billion tons of plastic has been manufactured during the last half a century, and most of it is non-biogradable. It has already spread to the very ends of the earth – demonstrated by what is known as the Great Pacific Garbage Patch, found between Hawaii and California.
Fed by spiralling currents, it is a vast floating collection of plastic bottles, polystyrene cups, fishing nets, six pack rings, punctured balloons, cellophane wrapping, plastic bags and assorted fragments. The Patch spans an area greater than the entire UK, but floating plastic – mostly washed from the land - has spread all across the ocean: in some regions there is six times more of it suspended in the water than plankton.
It is more than just an eyesore - the United Nations Environmental Programme estimates that getting tangled with or trying to eat plastic to be a serious cause of death among marine animals and birds. The same wave action that grinds stones into sand will gradually erode the plastic into progressively smaller fragments which will be potentially even more ingestible.
Eventually the plastic (perhaps much of it now caught up in the remains of once-living things) will settle at the bottom of the sea, compacted over time to form a unique sedimentary layer - ready for alien geologists to uncover.
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