This is the 1725th most frequent Chinese word.
地下
"Underground" or "subterranean."
In this sentence, '地下' is combined with '室' to form '地下室', which means 'basement'. It refers to a location below the ground level.
他把秘密文件藏在地下室里。
He hid the confidential documents in the basement.
Here, '地下' acts as a descriptor for '铁路系统' (railway system), meaning 'underground'. It emphasizes the location of the railway system beneath the ground surface.
城市的地下铁路系统非常发达。
The city's underground railway system is very developed.
In this instance, '地下' metaphorically refers to 'covert' or 'hidden', describing organizations operating in secrecy, not physically underground.