The Fountain outside the Entrance.

[MidnightMist]

I got the Antechamber Key on the third day, drained the fountain, went down...and there's nothing to do? I pushed the cart and there's some cog platforms that I can't access...beyond that there's nothing to do down here. I'm officially at a loss?

[Forblaze]

Moving the cart was important, but there's another place you can also use the antechamber key.

[Aiscence]

Originally posted by Forblaze:

Moving the cart was important, but there's another place you can also use the antechamber key.

Sadly they are not garanteed to have it yet, that's something that I m a bit iffy about, but that thing can appear after a few run but also a few hours ...