Sure, sewers and street-cleaning and road maintenance are all really important.  But have you ever really stopped to think about how important those sorts of services are? Have you ever imagined a world in which all those kinds of functions have ceased?

Alan Weisman has. He's an associate professor of journalism at the University of Arizona, and he's written a new book, "The World Without Us," which examines what would happen to mankind's cities if humans suddenly ceased to be.

The July issue of Scientific American profiles Weisman's ideas:

According to Weisman, large parts of our physical infrastructure would begin to crumble almost immediately. Without street cleaners and road crews, our grand boulevards and superhighways would start to crack and buckle in a matter of months. Over the following decades many houses and office buildings would collapse, but some ordinary items would resist decay for an extraordinarily long time. Stainless-steel pots, for example, could last for millennia, especially if they were buried in the weed-covered mounds that used to be our kitchens. And certain common plastics might remain intact for hundreds of thousands of years; they would not break down until microbes evolved the ability to consume them.

Weisman also lays out an extremely interesting timeline for the decay of the city of New York, which starts with the flooding of the city's subway system after only two days without humans. There's even a really cool video chronicling the changes.

So if you ever need a reminder of how important your street-cleaning job is, remember that you're the only thing standing between your city and utter annihilation.