Webster's English Dictionary

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underground

1. un.der.ground \.*n-d*r-'grau.nd\ av 1: beneath the surface of the earth  
   2: in or into hiding or secret operation 
2. un.der.ground \'*n-d*r-.grau.nd\ aj 1: being, growing, operating, or 
   situated below the surface of the ground  2: conducted by secret means 
3. underground \'*n-d*r-\ n 1: a subterranean space or channel  2: an 
   underground city railway system  3a: a movement or group organized in 
   strict secrecy among citizens esp. in  an occupied country for maintaining 
   communications, popular solidarity, and concerted resistive action pending 
   liberation 3b: a clandestine conspiratorial organization set up for 
   revolutionary or o ther disruptive purposes esp. against a civil order