Synonyms of the word

subterraneous

• adj. Subterranean.

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.

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.

subterranean

• adj. below ground, under the earth, underground.

ulterior

• adj. Situated beyond, or on the farther side.
• adj. Beyond what is obvious or evident.
• adj. Being intentionally concealed so as to deceive.
• adj. (archaic) Happening later; subsequent.