When Riis arrived in New York City, he was one of a large number of migrants and immigrants, seeking prosperity in a more industrialized environment, who came to urban areas during the years after the American Civil War. Twenty-four million people relocated to urban areas, causing their population to increase eightfold. The demographics of American urban areas became significantly more heterogeneous as many immigrants arrived, creating ethnic enclaves often more populous than many of the cities of their homelands. "In the 1880s 334,000 people were crammed into a single square mile of the Lower East Side, making it the most densely populated place on earth. They were packed into filthy, disease-ridden tenements, 10 or 15 to a room, and the well-off knew nothing about them and cared less."