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.
(..) Special Programs Department. George Bosch. Barbara Schlesinger. Tommy Pellegrino. l just saw footage of a special Bill's doing on the revolutionary underground. Most of it's tedious stuff of Laureen Hobbs and two fatigue jackets... muttering mutilated Marxism... but he's got eight minutes (..)1976 Network
(..) This is the Ecumenical Liberation Army. They're the ones who kidnapped Mary Ann Gifford three weeks ago. There's a lot of liberation armies in the revolutionary underground... and a lot of kidnapped heiresses. This is Mary Ann Gifford. That's the Great Ahmed Khan. He's their leader. (..)1976 Network