beneath the surface of the ground: traveling underground by subway.
in concealment or secrecy; not openly: subversion carried on underground.
adjective
existing, situated, operating, or taking place beneath the surface of the ground.
used, or for use, underground.
hidden or secret; not open: underground political activities.
published or produced by political or social radicals or nonconformists: an underground newspaper.
avant-garde; experimental: an underground movie.
critical of or attacking the established society or system: underground opinion.
of or for nonconformists; unusual: an underground vegetarian restaurant.
noun
the place or region beneath the surface of the ground.
an underground space or passage.
a secret organization fighting the established government or occupation forces: He fought in the French underground during the Nazi occupation of France.
(often initial capital letter) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.
Chiefly British. a subway system.
verb (used with object)
to place beneath the surface of the ground: to underground utility lines.
Italy may owe some of its seismic activity to carbon dioxide bubbling up from deep underground.
Carbon dioxide from Earth’s mantle may trigger some Italian earthquakes|Maria Temming|August 26, 2020|Science News
So the group of refuseniks — one of many underground cells plotting freedom — decided to hijack a plane.
The Failed Hijacking That Remade the Soviet Union|Eromo Egbejule|August 24, 2020|Ozy
Archaeologists had known since 1916 that some holes lurked underground.
Underground mega-monument found near Stonehenge|Avery Elizabeth Hurt|August 11, 2020|Science News For Students
Data from NASA’s Dawn orbiter, the study suggests, show signs that it may be harboring an ocean deep underground.
The dwarf planet Ceres might be home to an underground ocean of water|Neel Patel|August 11, 2020|MIT Technology Review
For example, the team found that a seismometer placed 380 meters underground near Auckland, New Zealand, was not only registering human activity, but saw that activity halved during the lockdowns.