Nine Orders of Magnitude
Forgive all the attributions here; I'm just giving credit where credit is due:
Sullivan guest-blogger Patrick Appel directs us (via Kottke) toward a post from the "God Plays Dice" blog (using facts originally crunched by fakeisthenewreal) on how if you look at Census block groups (essentially the smallest units you can do any kind of reasonable statistics on), population density in the United States varies over nine orders of magnitude.
In case you're wondering, the most densely populated block group is one in New York County, New York -- 3,240 people in 0.0097 square miles, for about 330,000 per square mile. The least dense is in the North Slope Borough of Alaska -- 3 people in 3,246 square miles, or one per 1,082 square miles. The Manhattan block group I mention here is 360 million times more dense than the Alaska one; population densities vary over a huge range.Wow! That's a huge difference.
I wondered where the two block groups in question were. I figured that a New York City block group that covered a little less than a hundredth of a square mile and contained more than 3,000 people would have to have a bunch of tall residential towers in it. Fakeisthenewreal identifies the b.g. in question as b.g. 1 of tract 279 in New York County (i.e., the borough of Manhattan). The easiest way to find this is to go to the American FactFinder's detailed tables for the 2000 Census, selecting "block group" under "geographic type," selecting the b.g. in question from the list, and then clicking "Map It."
You'll see that the b.g. in question is way up at the northern tip of Manhattan, in Washington Heights. It's the area bounded by 189th St., Wadsworth Terrace, Fairview Ave., Fort George Hill, St. Nicholas Ave., and Wadsworth Ave., which you can see on Google Maps here.
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If you then use Google's Street View function, you can take a look at how the area looks from the ground:The buildings aren't actually that tall. They look to be between six and eight stories. What they are, though, are very tightly packed. While there is a park across the street, within the block group there is almost no open space. There are no short buildings, and they're all apartments. Essentially, tenements. It's surrounded by a lot of similar development, but this particular b.g. is distinguished by having nothing else within its limits to add area without population or with lower population. The household size is pretty high, too, an average of 3.53, which you can see by checking table P17 for this b.g. on the FactFinder. It's pretty remarkable that this is more dense than areas with taller buildings.
The Alaskan block group is a big area on the Arctic Ocean west of Prudhoe Bay.
CORRECTION: I looked into the Alaskan block group in question a little more and determined that it is instead a big area on the Arctic Ocean that contains Prudhoe Bay. It seems that as of the last census, Prudhoe Bay only had a population of 5 persons. There are a lot more people there at any given time, but since they're mostly temporary oil workers and support personnel, they don't count in the census totals. I'll poke around and try to see if I can find the least-densely-populated block group that isn't overrun with temporary workers.