地下 dìxià underground

underground; subterranean; covert

How to remember 地下: 地 ground + 下 down = underground.

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地下 (dìxià) literally means underground/subterranean: 地下室 (dìxiàshì, basement), 地下车库 (underground garage).
It also has a figurative sense of "covert/clandestine": 地下工作 (dìxià gōngzuò, underground/secret work), 地下组织 (underground organization).
In some regions, especially Taiwan and southern speech, 地下 is used colloquially to mean simply "on the ground/floor" (东西掉在地下了 - it fell on the ground), overlapping with 地上 elsewhere in the Mandarin-speaking world.
Contrast with 地面 (dìmiàn, ground surface) - 地下 specifically stresses being below or beneath a surface.