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Underground

/ˌʌndəˈɡraʊnd/

Situated below the Earth's surface or done out of sight and in secret.

Synonyms:

The soldiers will discover the underground path.We are too far underground."Spike leads us through the winding tunnels of the secret underground.

Originating from the Middle English undergrund, combining under, meaning below, and ground, referring to the earth's surface.

10 mentions per 1 million words

Moderate

Hint: Frequency according to COCA