地下(dì xià) - underground, below ground in Mandarin

地下 (dì xià) is underground, and it works both as a place and as a modifier. On its own it is the region beneath the surface, 地下有水; in front of a noun it makes the underground version of it, 地下停车场, 地下铁, 地下室. Shops and lifts use it for basement levels, so 地下一层 is the first floor down.

The figurative sense came along the way English did. 地下 also means clandestine: 地下工作 underground political work, 地下钱庄 an illegal money exchange. Context does all the disambiguating, since nobody parks in a secret car park.

The trap is a tone shift. Read with a neutral second syllable, 地下 (dì xia) means on the ground or on the floor, which is the opposite of underground. 钱掉在地下了 means the money fell on the floor, not into the earth. Northern speakers use this a lot where southern speakers say 地上, so listen for the weak 下 rather than assuming the fixed word.