Jefferies tubes are the narrow crawlways that run through a starship, giving engineers access to systems and offering the crew an alternate route when turbolifts fail.
The tubes contain power conduits, control junctions and other components, and are lined with access panels and ladders between decks. Engineers spend much of their time in them, and crews frequently retreat into them when a ship is boarded or its main corridors are blocked. The cramped tubes have also served as hiding places, escape routes and spots for private conversation.
The name honors Matt Jefferies, the art director who designed the original Enterprise for the first series.