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Related to underground: Underground hip hop
1. To conceal oneself in a hidden place or at a hidden location so as to avoid discovery or detection, especially by figures of authority. Many political dissidents have gone underground now that the government has begun its violent crackdown on opposing parties. The agent had to go underground after his cover was blown.
2. To operate or function without being detected by someone or something, especially a body or figures of authority. The distributors of the pro-socialist pamphlets seem to have gone underground ever since they started attracting the attention of the feds.
1. capitalized An organized network of secret workers, routes, and safe houses used to ferry escaped African-American slaves to free states or present-day Canada. A former slave herself, Harriet Tubman was an instrumental figure in the Underground Railroad, saving roughly 70 people from slavery over the course of 13 rescue missions.
2. By extension, any network of people working together secretly to help fugitives escape to places of safety and freedom. The human rights organization has begun operating an underground railroad in the third-world country to help human trafficking victims escape from bondage. A former slave herself, Harriet Tubman was an instrumental figure in the Underground Railroad
A secret network for moving and housing fugitives, as in There's definitely an underground railroad helping women escape abusive husbands. This term, dating from the first half of the 1800s, alludes to the network that secretly transported runaway slaves through the northern states to Canada. It was revived more than a century later for similar escape routes.
in. to go into hiding; to begin to operate in secret. The entire operation went underground, and we heard no more about it.