You searched for: “subterraneous
subterraneous (adjective) (not comparable)
1. Concerning something that exists under the surface of the Earth or is situated within the ground: Jack found out that there were subterraneous, or underground, springs and subterraneous passages leading down to the springs near his town!
2. Pertaining to something that is beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed): In the novel, Tom's subterraneous motive for murdering his grandfather was not disclosed until the end of the last chapter.