From Longman Dictionary of Contemporary Englishthe Undergroundthe Undergrounda) British EnglishTTT a railway system under the ground SYN subway American English b) PPSECRETan illegal group working in secret against the rulers of a country → underground
Examples from the Corpus
the Underground• Did Hollywood and the underground film-makers, in particular, hold any responsibility for such behaviour?• If it is chosen, the underground site could start receiving canisters of waste in 2010, Olds said.• In 1971 the Pelham Street premises were threatened with demolition as London Transport wanted to re-develop the underground station.• Not that the early issues had learnt anything from the underground.• The environmental coalition also has proposed some type of barrier to prevent further expansion of the underground plume.• Among snatches of conversation, both of us remember his referring to the underground as being rather like hell. 