The tunnel is below the Golem Factory, but you need to reach the control panel in the southeast corner of the second floor to pass through it. Enter the factory, and retrieve a key from a chest under the factory; the staircase is in the northeast corner. When that is done, climb to the second level through a staircase in the northwest corner of the first, and head for the control panel. Once you have reached the control panel, press the grid alpha and beta buttons as well as the two shielding buttons. From there, go down to the lowest level, and then to the north exit.
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