/ˈʌndəɡraʊnd/ – adjective
Definition: ngầm, bí mật.
A more thorough explanation: In legal terms, “underground” typically refers to activities or operations that are conducted secretly, illegally, or without proper authorization. It can also refer to structures or facilities that are located beneath the surface of the ground, such as underground tunnels or storage facilities.
Example: The police discovered an underground drug trafficking operation in the abandoned warehouse.