underground air矿内空气undergroundadj.地下的；秘密的adv.在地下；秘密地n.地下；地铁；地道；秘密活动underground communication cable地埋通信电缆underground trade in weapons.秘密军火交易组织underground communication system矿井通信系统underground transmitting station地下发信台underground coal mining district井下采煤区underground garification station地下气化站Underground brine flow direction地下水流向underground water sealed caverns地下水封洞库underground gasification of coal煤炭地下气化underground pipeline engineering地下管道工程