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.