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.
(..) With our compasses. Okay, so the gate has a really strong electromagnetic field, and that can change the directions of a compass needle. Is this gate underground? Yes. Near a large water tank? Yes. How do you know all that? He's seen it. (..)2016 TvShow: Stranger Things Title: Chapter Seven: The Bathtub Season: 1 Episode 7