the underground economy
noun/ði ˌʌndəɡraʊnd ɪˈkɒnəmi/
/ði ˌʌndərɡraʊnd ɪˈkɑːnəmi/
(North American English) (British English the black economy)
[singular]- business activity or work that is done without the knowledge of the government or other officials so that people avoid paying tax on the money they earn
地下经济活动；黑市经济