Short of a shuttered toy factory, an empty zoo is perhaps one of the saddest abandonments one could encounter. But that, of course, hasn't kept away the more dedicated urban explorers.
If you're up for a visit to one yourself, Environmental Graffiti has compiled a number of such disused and aging zoos as captured by a number of intrepid photographers.
Two are outside the U.S., but the rest can be found in California, Massachusetts, Michigan, Missouri and Pennsylvania.