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The fore-chamber was a large underground chamber on Myst Island accessible from the dock. There is a circular imager on the floor of the room, which is able to play messages and display 3D images. The data on the imager is as follows: Topographical Extrusion Test, Water-Turbulence Pool, Marker Switch Diagram, and a message from Atrus to Catherine.
The fore-chamber also served as Rime's "place of protection." To access Rime, activate the Topographical Extrusion Test, then walk to the back of the imager, and press the button there. The control panel for the imager will open and fold down, revealing the device used to access Rime's storage chamber.