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.
(..) ... Zion's where the party would be. It's a city? The last human city. The only place we have left. Where is it? Deep underground near the Earth's core, where it's still warm. You live long enough, you might see it. Goddamn! I gotta tell you, I'm excited to see what you're capable of ... (..)1999 The Matrix