I'm assuming you found the "
first one"
easily enough, that is, the room that you need a yellow key to enter.
The second secret room can only be found once you are very far in the first half of the game. Spoiler on where it is and how to get there to follow:
Click here to view the secret text
×The secret room is Once East, and it can be accessed from the Entrance. You need the Grappling Hook to get there.
(And no, you can't go any further backwards. Tendry's journey must continue to the surface, not backtrack backwards!)
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Quick links to my stuff (in case you forgot where it was):
Click here to view the secret text