To get to Crater Island, take the mining car you'll find in a hole near the clear-cut trees on Jungle Island. The mining car will let you out near a lake with a large boiler nearby. Solving the Boiler Puzzle will give you access to Gehn's Laboratory and the Great Golden Dome to prep it one more time. From Gehn's Laboratory you can also take a tram to Plateau Island.
In Gehn's lab, you will find his journal, which reveals more of what Gehn is up to on Riven and also contains the five D'ni number code for opening the Fire Marble Domes. These numbers change for each game of Riven, supposedly, so there's not just one to give out here. After that, it's on to Plateau Island.
The Boiler
 |  | | Turn the valve to the center position to shift power to the boiler. |  | | These controls turn off the boiler, so you can get inside. |
This is the first puzzle you must solve when you arrive on the island. First, take the walkway out to the middle of the lake and turn the steam valve to the center position.
Go back to the boiler and walk to the side facing the lake and turn to face it. Here are all the controls for the boiler.
Make sure the power at the Y junction (lower left of the screen) is switched to the far right pipe. Turn the large wheel to move the pipe, and the water should drain from the boiler.
Then, set the Y junction valve to the lower branch (closest to the center). Throw the switch at the upper right, and a walkway grate (inside the boiler) should rise into position. Turn the valve on the pipe to the right to turn off the boiler. Now you can enter the boiler through the steel door around the other side. This will take you to Gehn's Laboratory.
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