Noun:
a secret group organized to overthrow a government or occupation forcean electric railway operating below the surface of the ground usually in a city;in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'
Adjective:
under the level of the ground;belowground storage areas
underground caverns
conducted with or marked by hidden aims or methods;clandestine intelligence operations
cloak-and-dagger activities behind enemy lines
hole-and-corner intrigue
secret missions
a secret agent
secret sales of arms
surreptitious mobilization of troops
an undercover investigation
underground resistance
Adverb:
in or into hiding or secret operation;the organization was driven underground
beneath the surface of the earth;water flowing underground
underground，subway，tube这些名词均可表示“地铁”之意。
underground一般用词，指城市中的地铁，主要用于英国。
subway指城市中的地铁或通道，主要用于美国。
tube主要用于英国口语中。
用作形容词 adj.
～+名词underground dustbin地下垃圾坑underground passages〔caves, water〕地下通道〔洞穴,水〕
近义词 buriedsecret反义词 overground地上的
用作形容词adj.
用作定语
～+ n.There is an underground car park.这里有一个地下停车场。
The spy never told his family about his underground activities.那个间谍从来不把他的地下活动告诉他的家人。
名词44%,副词42%,形容词14%
He is well known for his reports onundergroundcrime and gangs.他因为关于报道秘密的犯罪和帮派而出名。
The insurrectionary detachments started in Yunghsin as anundergroundforce,but they have come into the open since we captured the entire county.暴动队始于永新，原是秘密的，夺取全县以后，公开了。用作副词Moles stayundergroundmost of the time.钱鼠大部分时间住在地下。
Torrential rivers are rumbling deepunderground.急流在地下深处隆隆奔腾。
They were making a planunderground.他们在秘密地策划。
Enemy's spies workundergroundto collect our information.敌人的间谍秘密地收集我们的情报。用作名词Moles stayundergroundmost of the time.钱鼠大部分时间住在地下。
Torrential rivers are rumbling deepunderground.急流在地下深处隆隆奔腾。
I obtained it for one pound in theunderground.这是我在地铁花一英镑买来的。
The prisoner escaped by digging anundergroundwunnel.囚犯挖了一条地道逃跑了。
We were forced to moveunderground, to the tunnels here.我们被迫撤向地下，来到了这些地道里。
The spy never told his family about hisundergroundactivities.这位间谍从未把他的秘密活动告诉家里人。 They were undergrounding power lines.
他们正铺设地下电力线。《21世纪大英汉词典》