Look beyond the locked entrance
The passkey search leads through long branches of the Underground Depths. Searching only around the Undermoors door misses the routes you need to investigate.
Two branches to investigate
- Explore the upper route near a save pool in the Depths.
- Check the surface entrance left of the spacecraft for the other branch.
Follow these branches into the Depths instead of repeatedly returning to the locked gate. If you are carrying the key back through the worms-and-acid area, see the upper return-route hint.