tunnel, cave, catacomb, burrow, shaft, drift, adit, chunnel
An underground passage
Secret of the Underground Passage - 1911 was released on: USA: 14 October 1911
it means a secret passage
A passage through a hill ; underground, is called a TUNNEL. However, a passage between two hills (surface) is a VALLEY. A passage between two hills is dependent on its width. If the hills are low and far apart, then it is a VALE. However, if the hills are high and close together then it is a GLEN or RAVINE, or GORGE.
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's a cave.
it was a passage way for slaves to get to the north.
It's a small passage
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.