By Alex Arbuckle for Mashable

In the last decades of the 19th century, lower Manhattan was a densely packed collection of slums. With waves of immigrants entering the city and land at a premium, landlords bought up buildings and subdivided them into ever smaller partitions, housing dozens of people together in squalid, dark, unventilated rooms. Buildings often covered 90% of a standard 25-by-100-foot lot, with windows and ventilation only at the front and back. Read More