"Passageway" is one word.
The one word synonym for "underground passageway" is "tunnel".
Ramp
Strait
ramp
Passage.
Two nostrills
That is 'passageway' .
A strait
isthmus
There isn't one. But you can get one for your house.
Another word for tube or passageway is "duct." This term is often used in contexts such as anatomy, plumbing, or ventilation systems to describe a channel or conduit through which substances can flow. Other synonyms include "channel" and "conduit."
It is used as one word.