Why was it called the Underground Railroad?

No one is quite sure where the name "Underground Railroad" came from. Things that are underground are generally invisible. Because the operations of the Underground Railroad were secret, they were invisible to most people. Although slaves had been escaping for many years, the name was given to the network around the 1830s, at the same time that railroads were beginning to carry passengers across the United States. Because the routes of the escapes were a secret, it was as if the journeys were underground and out of sight.

The Underground Railroad used terms or codes taken from the language used on a real railroad. 

Other secret efforts have also had similar names. One example occurred during World War II when people who resisted the Nazis in Europe were called the Underground. Like the Underground Railroad, this network operated secretly to oppose the Nazis. Some members of this Underground helped Jews whom the Nazis wanted to kill. People hid Jews in their houses so the Nazi police would not find them. They sometimes helped them escape to a safe country where they were no longer in danger of being killed. The people of both underground movements put themselves at great risk to help others.