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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.

go underground

in. to go into hiding; to begin to operate in secret. The entire operation went underground, and we heard no more about it.
See also: underground