underground是可数名词。
underground：n.(城市的)地下铁路系统，地铁；秘密政治组织
Rescuers found victims trapped several feet underground.
营救人员发现有受难者被困在地下几英尺处。
The miners were trapped deep underground.
矿工被困在地下深处。
The escaped prisoners were eventually trapped in an underground garage and recaptured.
越狱逃犯终于给追逼到地下汽车库，再次被捕。
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