Synonyms of the word

underground

• adj. (not comparable) Below the ground; below the surface of the Earth.
• adj. Hidden, furtive, secretive.
• adj. Of music, art, etc, outside the mainstream.
• adv. Below the ground.
• adv. Secretly.
• n. (chiefly Britain) An underground railway.
• n. (with "the") A movement or organisation of people who resist political convention.
• n. (with "the") A movement or organisation of people who resist artistic convention.
• v. To route electricity distribution cables underground.

belowground

• adj. beneath the surface of the ground; underground.

clandestine

• adj. Done or kept in secret, sometimes to conceal an illicit or improper purpose.
• adj. (freemasonry, of a person or lodge) Not recognized as a regular member.

cloak-and-dagger

• adj. Marked by menacing furtive secrecy, often with a melodramatic tint or espionage involved.

covert

• adj. (now rare) Hidden, covered over; overgrown, sheltered.
• adj. (figuratively) Secret, surreptitious, concealed.
• n. A covering.
• n. A disguise.
• n. A hiding place.
• n. Area of thick undergrowth where animals hide.
• n. (ornithology) A feather that covers the bases of flight feathers.

hole-and-corner

• adj. Conducted in secret; clandestine.

hugger-mugger

• adj. Secret, clandestine, sly.
• adj. Disorderly, chaotic, confused.
• n. (archaic) Secrecy.
• n. Disorder.
• adv. Secretly.
• adv. Confusedly, in a muddle.

hush-hush

• adj. Secret, not spoken of (or spoken of only in hushed whispers between insiders).
• n. secrecy; covert operation.

metro

• n. An underground railway.
• n. A train that runs on such an underground railway.
• n. An urban rapid transit light railway.
• n. A train that runs on such a railway.
• n. A metropolitan area.
• adj. Metropolitan.

railroad

• n. (chiefly US and Canada) A permanent road consisting of fixed metal rails to drive trains or similar motorized…
• n. (chiefly US and Canada) The transportation system comprising such roads and vehicles fitted to travel…
• n. (chiefly US and Canada) A single, privately or publicly owned property comprising one or more such roads…
• n. (figuratively) A procedure conducted in haste without due consideration.
• v. (transitive) To transport via railroad.
• v. (intransitive) To operate a railroad.
• v. (intransitive) To work for a railroad.
• v. (intransitive) To travel by railroad.
• v. (intransitive) To engage in a hobby pertaining to railroads.
• v. (transitive) To manipulate and hasten a procedure, as of formal approval of a law or resolution.
• v. (transitive) To convict of a crime by circumventing due process.
• v. (transitive) To procedurally bully someone into an unfair agreement.
• v. (role-playing games) To force characters to complete a task before allowing the plot to continue.
• v. (upholstery) To run fabric horizontally instead of the usual vertically.

railway

• n. (chiefly Britain, Ireland and Commonwealth of Nations) A transport system using rails used to move passengers…
• n. (chiefly Britain, Ireland and Commonwealth of Nations) A track, consisting of parallel rails, over which…

resistance

• n. The act of resisting, or the capacity to resist.
• n. (physics) A force that tends to oppose motion.
• n. (physics) Shortened form of electrical resistance.
• n. An underground organisation engaged in a struggle for liberation from forceful occupation; a resistance…

secret

• n. (countable, uncountable) Knowledge that is hidden and intended to be kept hidden.
• n. (uncountable) Something not understood or known.
• n. (archaic, in the plural) The genital organs.
• adj. Being or kept hidden.
• adj. (obsolete) Withdrawn from general intercourse or notice; in retirement or secrecy; secluded.
• adj. (obsolete) Faithful to a secret; not inclined to divulge or betray confidence; secretive.
• adj. (obsolete) Separate; distinct.
• v. (transitive) To make or keep secret.
• v. (transitive) To hide secretly.

subsurface

• n. Something that is below the layer that is on the surface.
• n. (countable, mathematics) A surface which is a submanifold of another surface.
• adj. below the surface.

subway

• n. (Canada, US, Scotland) An underground railway, especially for mass transit of people in urban areas.
• n. (US) A rapid transit system, regardless of the elevation of its right of way.
• n. (Britain) An underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
• v. (intransitive, US, informal) To travel by underground railway.

surreptitious

• adj. Stealthy, furtive, well hidden, covert (especially movements).

tube

• n. Anything that is hollow and cylindrical in shape.
• n. An approximately cylindrical container, usually with a crimped end and a screw top, used to contain and…
• n. (Britain, colloquial, often capitalized as Tube) The London Underground railway system, originally referred…
• n. (Australia, slang) A tin can containing beer (or other beverage?).
• n. (surfing) A wave which pitches forward when breaking, creating a hollow space inside.
• n. (Canada, US, colloquial) A television. Also, derisively, boob tube. British: telly.
• v. To make or use tubes.

undercover

• adj. Performed or happening in secret.
• adj. Employed or engaged in spying or secret investigation.
• n. A person who works undercover.