The phrase "secret tunnel" refers to a hidden passage or pathway that is not easily seen or known to most people. It suggests something hidden or secretive, often used in stories or adventures. Full definition
Though he managed to escape through secret tunnels, he was later caught and cut into pieces. (thetravelmagazine.net)
The mysterious caves and secret tunnel entrance, its gentle but rugged cliffs and abundant marine life all compliment mother nature. (anothersideofsandiegotours.com)
Have you ever wondered what secret tunnels and buildings are hidden under the city streets? (artlyst.com)