It's a cave.
An underground passage
Adit
Secret of the Underground Passage - 1911 was released on: USA: 14 October 1911
it means a secret passage
There is an underground passage in Washington DC. This is in place in case of an emergency so that the President and important staff can go there if the need arises.
it was a passage way for slaves to get to the north.
A river that flows into an underground passage or sinkhole.
the one to the left of lavender town
"The Underground Railway"
Simply go inside then go downstairs.
"Underground" is typically written as one word, both as an adjective (e.g., underground passage) and a noun (e.g., subway system).
The underground railroad isn't actually a railroad or underground, its actually just a secret passage where African American slaves used to escape slavery. They were usually smuggled in coffins!