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.
(..) I wanted to tell you that I was running errands at the mall. You know, the new mall that's just opened. A mini-mall but with underground parking. What do they call those? I pulled in to see what they had done with it and lo and behold they had a video store and... I got Silent Running. (..)2003 TvShow: Six Feet Under Title: Making Love Work Season: 3 Episode 6