Underground means below the surface of the earth. Cables buried underground, a car park underground, a stream that goes underground for half a mile and comes back up in the middle of a field.
In London the Underground is the railway beneath the city — capital U, and everybody calls it the Tube; Paris has the metro, New York the subway. The word also works nowhere near soil: an underground newspaper, an underground band, an underground movement are all hidden from the authorities, or outside the mainstream and pleased about it, and in wartime the underground was the resistance. It shifts part of speech without changing shape — the pipes run underground, an underground station, take the Underground home. And to go underground is to disappear from view on purpose.
1adjectiveburiedBelow the surface of the ground.~30%
“The cables run underground for two miles.”
synonyms: buried
2nounrailThe railway that runs beneath a city.~29%
“Take the Underground as far as Victoria.”
synonyms: subway, metro
3adjectiveundergroundOutside the mainstream, or hidden from the authorities.~41%
“An underground magazine passed from hand to hand.”
Shares are an estimate: a language model read 176 sentences found in the wild and placed 136 of them on a sense (40 were unclear) — a count of our sample, not a measurement of English.
Middle English
undergrounde
English, today
underground
From Middle English undergrounde. From Middle English undergrounde, equivalent to under + ground or under + ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds.
Etymology from Wiktionary, CC BY-SA