To dig underneath (something), to make a passage for destructive or military purposes; to sap. / (figuratively) To weaken or work against; to hinder, sabotage.

(not comparable) Below the ground; below the surface of the Earth. / (figuratively) Hidden, furtive, secretive.

Below the ground. / Secretly.

(geography) Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines). / (chiefly Britain) Synonym of subway: a railway that is under the ground.

To route electricity distribution cables underground.

A competitor thought unlikely to win. / Somebody at a disadvantage.

To strengthen, secure, or reinforce by passing a rope, cable, or chain around the underside of an object.

Performed or happening in secret. / Employed or engaged in spying or secret investigation.

A person who works undercover.

To provide too little coverage.