underground

underground

I

un‧der‧ground¹ /ˈʌndəɡraʊnd $ -ər-/ adjective1. below the surface of the earth:
    an underground passage
    The car park is underground.
2. [only before noun] an underground group, organization etc is secret and illegal:
    an underground terrorist organization
3. [only before noun] underground literature, newspapers etc are read by a small number of people, and would seem slightly strange or shocking to most people:
    the underground press
     

THESAURUS

   ▪ secret known about by only a few people, who have agreed not to tell anyone else: a secret meeting place | The details of the proposal must remain secret.
   ▪ confidential used about information, especially in business or government, that is secret and not intended to be shown or told to other people: a highly confidential report | Employees’ personal details are treated as strictly confidential.
   ▪ classified used about information that the government has ordered to be kept secret from most people: He was accused of passing on classified information to the Russians in the 1950s.
   ▪ sensitive used about information that is kept secret because there would be problems if the wrong people knew it: A teenager managed to hack into sensitive US Air Force files.
   ▪ covert [only before noun] used about things that are done secretly, especially by a government or official organization: a CIA covert operation
   ▪ undercover [usually before noun] used about things that are done secretly by the police in order to catch criminals or find out information: Detectives arrested the suspect after a five-day undercover operation.
   ▪ underground an underground organization or newspaper is one that operates or is produced secretly and opposes the government: Her father was a member of the underground resistance movement in France during World War II.
   ▪ clandestine /klænˈdestən, klænˈdestɪn/ secret and often illegal or immoral: clandestine meetings | his involvement in a clandestine operation to sell arms to Iran | a clandestine love affair
   ▪ hush-hush informal used about information or activities that are kept officially secret: He was put in charge of some hush-hush military project. | I’ve no idea what he does – it’s all very hush-hush.

II

un‧der‧ground² /ˌʌndəˈɡraʊnd $ -ər-/ adverb1. under the earth’s surface:
    This animal spends most of its life underground.
    nuclear waste buried deep underground
2. go underground to start doing something secretly, or hide in a secret place:
    The ANC was forced to go underground when its leaders were arrested.

III

un‧der‧ground³ /ˈʌndəɡraʊnd $ -ər-/ nounthe Underground
  a. British English a railway system under the ground
   SYN  subway American English
  b. an illegal group working in secret against the rulers of a country