underground Synonyms

secret belowground hugger-mugger cloak-and-dagger hole-and-corner hush-hush undercover clandestine surreptitious underground railway behind-the-scenes under-the-table into hiding black-market passenger train maglev resistor back channel public transport avant-garde under cover underneath undergrow underfoot tube train maquis maquisard gray market top secret rolling stock resistivity on the qt subsurface MRT London Underground underground railroad under the counter fifth column undisclosed below ground saboteur unconventional in the earth metro public transit undersoil alternative covert dern