The political party 'went underground' [=started working in secret] after the new government took power.
I've ridden on the New York subway, the Paris Metro, and the London 'Underground'.
The drugs are supplied through an 'underground' network.
(..) Oh,... this is so humiliating. Try not to make a sound. What are you telling him Mufasa? Mufasa?...Simba? Ha ha ha ha ha. That's very good. Ha ha ha... Zazu. Yes? Sir. News from the underground. Now, this time-- Sire! Hyenas!! In the Pride Lands! Zazu, take Simba home. Oh Dad, can't I come? (..)1994 The Lion King