Re: Detroit
People go where the jobs are. The auto industry built up there a hundred years ago because it was well situated for the import of raw materials and the export of autos by the great lakes and the rail system, as transport was the major cost to construction. Now, the dominant cost is labor, and there's less incentive for the auto manufacturers to centralize their production facilities. As a result of that, the increased automation of production, the rise of imports, and a lack of new industries, there are not enough jobs to support the number that once lived there.
As a result, people have left to where the jobs are, leaving large swaths of Detroit abandoned. The city doesn't have enough money to deal with all of these, and no one wants to move to Detroit (for the reasons mentioned above), so the lots just rot.
I don't think there's a solution to bring Detroit back to it's "glory years", but I do think we should spend some money to level a lot of the abandoned regions and return them to fields, and let Detroit begin anew as a smaller city. I don't think that'll happen, though.