MUCH OF Detroit is in ruins. The city spreads over 139 square miles, but about 20 square miles of that — an area just under the size of Manhattan — is vacant. Home in 1950 to more than 1.8 million people, today Detroit has less than half that many residents. There are tens of thousands of abandoned homes and buildings that are crumbling and decaying in the metropolis, and the city is demolishing these structures because they can be, among other things, good hideaways for people with drug addiction and places to dump the bodies of murder victims. Since 2014, teams have razed more than 18,000 buildings, with thousands of demolitions still to come.