The Southern Underground Railroad

This map, via Smithsonian, shows the main passageways that the enslaved took southwards to freedom.

The southern Underground Railroad was less traveled and much less well documented, but Mexico after gaining independence passed strong anti-slavery laws. Even before that the Spanish government granted freedom to many who escaped to the south. When the United States passed the Fugitive Slave Act in 1850, Mexico refused to comply despite pressure from US diplomats. When Mexico lost Texas and large portions of other states to the United States as a result of the Mexican-American War, slavery was re-established in Texas and the Rio Grande became the crossing to freedom. - via Smithsonian