Frequency: The word is one of the 3000 most common words in English.
1.
Something that is ~ is below the surface of the ground.
Solid low-level waste will be disposed of deep ~...
The plane hit so hard that one engine was buried 16 feet ~.
ADV: ADV after v
•
Underground is also an adjective.
...a run-down shopping area with an ~ car park.
...~ water pipes.
ADJ: ADJ n
2.
The ~ in a city is the railway system in which electric trains travel below the ground in tunnels. (BRIT; in AM, use subway )
...a woman alone in the ~ waiting for a train...
He crossed London by ~...
N-SING: the N, also by N
3.
In a country which is controlled by an enemy or has a harsh government, the ~ is an organized group of people who are involved in illegal activities against the people in power.
These US dollars were smuggled into the country during the war, to aid the ~.
N-SING: the N
4.
Underground groups and activities are secret because their purpose is to oppose the government and they are illegal.
They are accused of organising and financing an ~ youth movement.
ADJ: ADJ n
5.
If you go ~, you hide from the authorities or the police because your political ideas or activities are illegal.
After the violent clashes of 1981 they either went ~ or left the country.
ADV: ADV after v