Wikipedia: Underground railroad

The Underground Railroad was the name of a network of clandestine routes, often informal and impromptu, by which slaves were able to escape the United States and reach freedom in Canada. The underground railroad consisted of secret safe houses and other facilities owned by anti-slavery sympathizers, and operated much like any other large-scale widespread resistance movement with independant cells that only knew of a few of their neighbours. Escaped slaves would pass from one way station to another, making their way north step by step.