subterranean

adj

1. (not-comparable)Below ground, under the earth, underground.

   “Again the bearers took up the coffin, and cold and damp the subterranean air came from the opened vault. The tapers were lowered, and shed a ghastly light on the rows of piled coffins, and the moisture glittering on the walls.”

   “And you were in the parking lot / Subterranean by your own design”

   “This is a story of north-south connection, and it begins with the fact that late in 1863 the Great Northern Railway completed a subterranean connection from its terminus at King's Cross to the tracks of the Metropolitan enabling the Great Northern to run through to the Met's easterly terminus at Farringdon Street.”

   “The best way to neutralise its [nuclear waste's] threat is to move it into a subterranean vault, of the kind the UK plans to build later this century. Once interred, the waste will be left alone for tens of thousands of years, while its radioactivity cools.”

   “Today is that most sacred of American holidays. A day when people from all walks of life set aside their differences and focus their undivided attention on the prognostication abilities of a subterranean rodent.”

2. (broadly, not-comparable)Secret, concealed.

   “The subterranean tool of buying rigged opinion polling and media coverage is outlined in remarkable detail in chat exchanges recovered from the cellphone of one of Mr. Kurz’s closest allies and friends, Thomas Schmid.”

noun

1. Someone or something that is subterranean.

   “[H]e continues Lean like a Hawke or Heron, notwith ſtanding his devouring Appetite: yet it would ſeem that they convey that ſubſtance elſewhere, for theſe Subterraneans eat but little in their Dwellings; […]”

   “Whatever elſe the Tauric Cimmerium have been right or wrong called, it is no leſs ſurely a capital of darkneſs, than, near the famous Baths of Baiae, in Campania, the Cimmerium, where the inhabitants were truly Cimmerian; as miners, or other ſubterraneans; who, for good or evil purpoſe, never faced the ſun.”

   “Prue found an ample subterranean, neatly furnished; […]”

   “Above: a stretch of the walls of the old Cathedral of S. Reparata in the subterraneans of the Duomo.”