The Underground Railroad was a network of secret routes and safe houses established in the United States during the early to mid-1800s, and used by enslaved African-Americans to escape into free states and Canada. The scheme was assisted by abolitionists and others sympathetic to the cause of the escapees. Not literally a railroad but rather a secretly organized means of escape, the workers both black and white, free and enslaved, who aided the fugitives can also be referred to as the "Underground Railroad". Various other routes led to Mexico, where slavery had been abolished, or overseas. An earlier escape route running south toward Florida, then a Spanish possession (except 1763–83), existed from the late 17th century until Florida became a United States territory in 1821 (ending the safe haven for escaped slaves was the main reason Florida changed nationality). However, the network now generally known as the Underground Railroad was formed in the late 1700s. It ran north and grew steadily until the Civil War began. One estimate suggests that by 1850, 100,000 slaves had escaped via the "Railroad".