In a fascinating piece of imagination and scientific inquiry, Alan Weisman describes what would happen if we humans suddenly weren't here.
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.
“But what would be happening underground? Corrosion. Just think of the subway lines below Lexington Avenue. You stand there waiting for the train, and there are all these steel columns that are holding up the roof, which is really the street. These things would start to corrode and, eventually, to collapse. After a while the streets would begin cratering, which could happen within just a couple of decades. And pretty soon, some of the streets would revert to the surface rivers that we used to have in Manhattan before we built all of this stuff. [More] [Update - Sorry, the link wasn't working. All better now.]
His book - A World Without Us - is not a scold about why we are doomed to extinction, just a science fiction experiment in what would happen. And I gotta admit, I have speculated myself on what this piece of the globe would look like without farmers like me grooming and repairing constantly.
Exactly why this captivates me I'm not sure. End-of-the-world scenarios are a part of our literature and religion (e.g. The Rapture). Perhaps it stems from our deeply held conceit the world just can't get along without us.