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.
(..) You'll only be a hindrance to them. Thank you. Shit. Jesus, it's coming. It's coming! It's headed right for us! It's already here. Everybody, underground! Honey! Come on! Jo, come on! Get underground! Take cover, now! (..)1996 Twister