1) Covert 2) Subsurface 3) Subterranean 4) Ulterior 1) Subsurface 2) Subterranean 
• (a.) Being or lying under the surface of the earth; situated within the earth, or under ground; as, subterranean springs; a subterraneous passage.
subterraneous 1. Being or lying under the surface of the earth; situated within the earth, or under ground; as, subterraneous springs; a subterraneous passage. 2. Lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed); such as, subterraneous motives for murder.