Level 4 Archives - Trails of Thought Gear Key Secret

[alusair1]

I am at the button which as I've read elsewhere is supposed to disable the "Crossroads" portal and open the trapdoor that I can fall in to get to the Gear key. However, after pressing the button, the portal stays active. I have noticed that two trapdoors close in the southeast corner of the room but looking at the map I know I need to fall in that middle trapdoor. Any thoughts?

[Ziel]

There's another button on the wall over the pit where the trapdoors close.