The number of Americans living in slums increased 33% in the past decade.

This scary trend was identified by a report out this morning from Brookings. Slums (our term) refer to areas of concentrated poverty where at least 40 percent of individuals live under the poverty line.

In the 1990s, America’s slum population decreased from 9,101,622 to 6,574,815. Many neighborhoods experienced gentrification, which saw poor people move out of, say, Brooklyn’s Park Slope while Starbucks moved in and property prices soared.

From 2000 to 2009, however, the slum population climbed back to 8,735,395 (since then it probably rose even higher). A full 747 census tracts returned to slum status.

Click on to see the cities with the greatest increases in concentrated poverty. The South and the Midwest are leading the collapse.