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.
(..) - Trevor. Behind you. - What? It is wet. It's cold. There's water behind here. There's gotta be water behind some pocket or underground river. Trevor, it's getting closer. Hey, that's magnesium. - We're slipping. - How many flares you got left? Three. (..)2008 Journey to the Center of the Earth
(..) ...a century ago. Hey, Trevor. I think I found Liedenbrock's notebook. Look. Looks like on the other side of those trees there's some sort of underground ocean. Yeah. I don't think that's Liedenbrock's handwriting. It's my dad's. Trevor? (..)2008 Journey to the Center of the Earth