crawlway

/ˈkrɔlˌweɪ/
noun
  1. A narrow passage or tunnel that is only large enough for a person to crawl through, often used for access in buildings, caves, or machinery spaces.
    • The plumber had to squeeze through a dark crawlway under the house to reach the leaky pipe.
    • The spaceship's maintenance crawlway was so tight that only the smallest crew member could fit inside.
    • In the old mine, the miners used a narrow crawlway to reach the deepest tunnels.
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