## Meanings
1. Below the surface of the ground.
2. Below the surface of the ground.
3. A city railway system that runs below the ground.
4. Secret or hidden, especially from the authorities.

## Examples
- There is an underground tunnel that goes across the river.
- The pipes run underground for several miles.
- In London the train system is called the underground.
- These criminals operate through an underground network.

## Roots and Affixes
- Root: undergrounde
- Affix: under- - below

## Etymology
From Middle English undergrounde (adverb), equivalent to under + ground or under- + ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds.

## AI Etymology
Underground is from Middle English undergrounde (adverb), equivalent to under + ground or under- + ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds. The historical line also includes Middle English undergrounde. In present-day use, it can mean below the surface of the ground, or a city railway system that runs below the ground.

## Topics
- Space, Position & Direction

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underground

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