underground

Compared meaning: located below the surface of the ground (adjective).

比较义项：地下的（形容词）。

• underground 本词

  The common, everyday word for anything below ground; used in ordinary contexts like parking, tunnels or pipes, and can also work as a noun or adverb, or mean 'secret/hidden' (senses not compared here).

  这是表示“在地下”的最常用、日常用词；用于停车场、隧道、管道等普通语境，也可作名词或副词，或表示“秘密的、地下组织的”（这些义项本卡片不作比较）。

  The mall has an underground parking garage for shoppers.

  这家商场有一个供顾客使用的地下停车场。

• subterranean

  A more formal, literary or technical word for underground, typically used in geological, scientific or descriptive writing rather than everyday speech.

  语体更正式、书面化，多用于地质、科学或文学性描写，较少用于日常口语。

  Geologists discovered a vast subterranean lake beneath the desert.

  地质学家在沙漠地下发现了一个巨大的地下湖。

Underground and subterranean both mean below the ground and are often interchangeable, but underground is the common, everyday word, used for things like parking or tunnels. Subterranean is more formal and literary, common in geological, scientific, or descriptive writing, not everyday speech.

这两个词在这个意义上都表示“地下的”，很多时候可以互换，但语体不同：underground是最常用、口语化的词，日常事物（停车场、隧道、地铁）都用它；subterranean更正式、书面化，常见于地质、科学或文学性描写。underground还有其他常见义项（如“秘密的、地下组织的”，以及作名词或副词的用法），这里不作比较。

Compared meaning: done secretly, hidden from others (adjective).

比较义项：秘密地进行、不为人知或不公开（形容词）。

• secret

  The most general word: describes information, plans or feelings kept hidden from most people; used for everyday, personal or small-scale secrecy, not necessarily organized or official.

  最常用、最泛化的词：指对大多数人隐瞒的信息、计划或感受；用于日常、个人或小范围的保密，不一定涉及组织或官方行动。

  She kept the recipe secret so no other bakery could copy it.

  她把配方保密，不让其他面包店抄袭。

• undercover

  Describes a person, often police or a spy, who disguises their identity to gather information from inside a group; almost always used for agents or investigators, not plans in general.

  指某人（通常是警察或间谍）伪装身份，打入某个团体内部获取情报；几乎只用于描述特工或调查人员，而不是泛指的计划。

  The detective went undercover as a waiter to catch the drug dealers.

  那名警探伪装成服务员卧底，去抓那些毒贩。

• underground 本词

  Describes activities, movements or organizations that operate secretly to avoid detection by authorities, often political or resistance-related; implies an organized network, not just one hidden fact.

  指为躲避当局察觉而秘密运作的活动、运动或组织，常与政治或反抗行动有关；暗示存在一个有组织的网络，而不只是一个被隐瞒的事实。

  The rebels ran an underground newspaper to avoid the government's censorship.

  反抗者秘密创办了一份地下报纸，以躲避政府的审查。

• covert

  A formal word for actions or operations deliberately planned to stay hidden and deniable, common in military, intelligence or government contexts; rarely used for personal secrets.

  较正式的用词，指经过策划、旨在保持隐蔽并可否认的行动或行动方案，常见于军事、情报或政府语境；很少用于个人的秘密。

  The general authorized a covert operation to rescue the hostages quietly.

  将军批准了一次秘密行动，悄悄营救人质。

Secret, undercover, covert, and underground all mean secretly done. Secret is the general word, for information hidden from people. Undercover describes a person, often police, hiding their identity in a group. Underground describes organized activities avoiding authorities. Covert is formal, for military or intelligence operations.

这四个词都表示“秘密进行、不公开”，但语气和使用场合不同：secret 泛指对多数人保密的信息或计划；undercover 强调伪装身份、打入某个团体内部调查；underground 强调为躲避当局监管而进行的地下活动；covert 语气正式，多用于军事或情报行动。这里不比较 underground“地下的，在地面以下的”这一字面义项。