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 African-American slaves to escape into free states and Canada with the aid of abolitionists and allies who were sympathetic to their cause. The term is also applied to the abolitionists, both black and white, free and enslaved, who aided the fugitives. Various other routes led to Mexico 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, and it ran north to the free states and Canada, and reached its height between 1850 and 1860. One estimate suggests that by 1850, 100,000 slaves had escaped via the "Railroad".