1. A hidden underground passage or corridor concealed from public view, often used for clandestine purposes
The castle had a secret tunnel that led to the forest.
O castelo tinha um túnel secreto que levava à floresta.
2. An underground passage used for smuggling, escape, or covert military operations
Prisoners escaped through a secret tunnel beneath the prison walls.
Os prisioneiros escaparam por um túnel secreto sob as muralhas da prisão.