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The doorway extends into a tunnel about 20 meters long, with a few doors on either side. All of these doors appear to be conventional, with handles in them.
Go back to the first room
Open the first door on the left-hand side
Open the second door on the left-hand side
Open the third door on the left-hand side
Open the first door on the right-hand side
Open the second door on the right-hand side
Open the third door on the right-hand side
This is part of The Author's The Game interactive book