July 16, 2025 2:31 AM (GMT+2) I suspect this door connects to the Rotating gear room. That’s because it really looks like it would lead to this specific entrance:

To open it, I suspect I need to insert the two remaining chips in their slots (see A small data microchip). I have no clue where they might be, though.

July 24, 2025 1:57 AM (GMT+2) Actually, that tunnel is not connected to the Blackbridge Grotto. It leads to the Inner Sanctum.

August 16, 2025 9:04 PM (GMT+2) The 15th letter from Randolph confirms that I need to insert the two remaining chips. It also hints that their location might be related to A New Clue. It also has a picture of a hammer, so perhaps the hammer is involved in finding the chips.

August 19, 2025 9:14 PM (GMT+2) I’ve found one of the two remaining chips (see Microchip buried besides the west bridge). The other is likely also related to [Meaning of A New Clue ](https://k34.notion.site/Meaning-of-251e98005eb080cbbc62e59397f4c16c), but I still haven’t figured it out.

August 24, 2025 4:32 PM (GMT+2) In fact I don’t think the other is related to [Meaning of A New Clue ](https://k34.notion.site/Meaning-of-251e98005eb080cbbc62e59397f4c16c), unless I’m not seeing something. The last clue was in relation to the Sundial at apple orchard, which allowed me to open it (see Opened sundial), revealing three spots in which I can also insert the chips. So, whenever I find the remaining chip, I’ll be able to use them in both places.

September 3, 2025 11:07 PM (GMT+2) I finally found the last microchip. A New Clue (draft) made it more clear, although it was still a bit obscure (a friend nudged me into the right direction). Turns out it was inside a pot in the Entrance Hall (a pot which, to my mind, looked like an oil lamp in the drawing).

In any case, the door has now been open, revealing a path to the Orindian Ruins.

I’ve also put the microchips in the Opened sundial and unlocked the Satellite Dish.

Mystery solved!