A maintenance shaft was an area in a starship where access to the ship's computer systems from computer terminals was possible.
In 2153, the two Borg-assimilated Tarkaleans used a ladder from sickbay to get into maintenance shaft C, junction 12. There they removed several wall panels and accessed the avionics control system and the warp plasma regulators.
A security team led by Lieutenant Malcolm Reed tried to stop the Tarkalean female but was attacked by the Tarkalean male. Captain Jonathan Archer saw no other option than decompress the area and blew the two through a hatch into space. (ENT: "Regeneration")