The backstairs (stairs at the rear of a building).
clandestine, secret or furtive
An underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
An underground route for pipes, sewers, etc.
A rapid transit system, regardless of the elevation of its right of way; a metro system.
An underground railway, especially for mass transit of people in urban areas.
A train that runs on such an underground railway.
To travel by underground railway.