“We are not evacuating Rikers Island,” Mayor Michael Bloomberg said in a news conference this afternoon. Bloomberg annouced a host  of extreme measures being taken by New York City in preparation for the arrival of Hurricane Irene, including a shutdown of the public transit system and the unprecedented mandatory evacuation of some 250,000 people from low-lying areas. But in response to a reporter’s question, the mayor stated in no uncertain terms (and with more than a hint of annoyance) that one group of New Yorkers on vulnerable ground will be staying put.

New York City is surrounded by small islands and barrier beaches, and a glance at the city’s evacuation map reveals all of them to be in Zone A (already under a mandatory evacuation order) or Zone B–all, that is, save one. Rikers Island, which lies in the waters between Queens and the Bronx, is not highlighted at all, meaning it is not to be evacuated under any circumstances.

According to the New York City Department of Corrections’ own website, more than three-quarters of Rikers Island’s 400 acres are built on landfill–which is generally thought to be more vulnerable to natural disasters. Its ten jails have a capacity of close to 17,000 inmates, and normally house at least 12,000, including juveniles and large numbers of prisoners with mental illness–not to mention pre-trial detainees who have yet to be convicted of any crime.