Wordnet 3.0
ADJECTIVE (2)
1. 
 under the level of the ground; 
- Example: "belowground storage areas"- Example: "underground caverns"[syn: belowground, underground]
2.  underneath the ground; 
- Example: "most of his friends are now belowground"
WordNet (r) 3.0 (2006):
belowground
    adj 1: under the level of the ground; "belowground storage
           areas"; "underground caverns" [syn: belowground,
           underground]
    2: underneath the ground; "most of his friends are now
       belowground"