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The key is located in the bathtub with a King Boo themed inner tube. Luigi can't get through the door since it's blocked by towels, he will have to go go the long way, avoiding lasers and get to the power source to disarm the lasers. Gooigi can drop down the grate in front of the shower, then go in the alcove opposite where there is a King Boo shadow (which is actually the gem), then go through the pipe to reach the blue gem. Two Golden Goobs can be seen reading a newspaper, which Luigi can capture to complete this achievement.