subterranean

/ˌsʌbtəˈreɪniən/
adjective
  1. Existing, happening, or done under the surface of the earth.
    • Moles are subterranean animals that spend most of their lives underground.
    • The city has an extensive subterranean network of tunnels and sewers.
    • Subterranean rivers can carve out huge caves over thousands of years.
  2. Hidden or secret, especially in a way that is not obvious.
    • His subterranean ambitions slowly came to light after years of quiet planning.
    • There was a subterranean current of anger beneath the calm discussion.
    • The organization operated through a subterranean network of informants.