If a virulent virus, or even the Rapture, depopulated Earth overnight, how long before all trace of humankind vanished?
There are places in Manhattan where they’re constantly fighting rising underground rivers that are corroding the tracks. You stand in these pump rooms, and you see an enormous amount of water gushing in. And down there in a little box are these pumps, pumping it away. So, say human beings disappeared tomorrow. One of the first things that would happen is that the power would go off. A lot of our power comes out of nuclear or coal-fired plants that have automatic fail-safe switches to make sure that they don’t go out of control if no humans are monitoring their systems
Once the power goes off, the pumps stop working. Once the pumps stop working, the subways start filling with water. Within 48 hours you’re going to have a lot of flooding in New York City. Some of this would be visible on the surface. You might have some sewers overflowing. Those sewers would very quickly become clogged with debris—in the beginning the innumerable plastic bags that are blowing around the city and later, if nobody is trimming the hedges in the parks, you’re going to have leaf litter clogging up the sewers.
Image credits: worldwithoutus.com
Many of the buildings in Manhattan are anchored to bedrock. But even if they have steel beam foundations, these structures were not designed to be waterlogged all the time. So eventually buildings would start to topple and fall.
At the other end of the chronological spectrum, anything made of bronze might survive in recognizable form for millions of years—along with one billion pounds of degraded but almost indestructible plastics manufactured since the mid-20th century.
Image credits: National Geographic
Meanwhile, land freed from mankind’s environmentally poisonous footprint would quickly reconstitute itself, as in Chernobyl, where animal life has returned after 1986′s deadly radiation leak, and in the demilitarized zone between North and South Korea, a refuge since 1953 for the almost-extinct goral mountain goat and Amur leopard.