This page includes Major gameplay spoilers or secrets. Tread carefully.
The Cauldron Room is one of two secrets in the game that can only be seen when playing without any mods enabled. The other is the Eye Messages.
It is located to the east of Hiisi Base's top-edge, and below the Sauvojen Tuntija's chasm. It is surrounded by extremely dense rock (EDR). It is unknown if digging is the proper way to get inside the room.
The room is surrounded by rock and is full of gray sand. In the middle of the room there is an altar with a cauldron on it, surrounded by pillars. Those three structures are made of EDR. The sand is non-magical (as opposed to the rock from which the stone shroom is built). In the background there is a giant stone tablet. It is worth noting that all of it has a custom VFX mask that doesn't disappear after material shifts (including Fungal Reality Shift or turning the world to gold).
Once the area has been found, reloading the game with mods enabled will cause the great stone tablet to vanish, but the room and cauldron remain.
When breaking into the room through digging, what is inside the "cauldron" varies based on the date. It will contain either Void Liquid - instantly breaking the cauldron and surrounding area - or nothing.
Currently, few details are known about the purpose of the room or the cauldron. Minä are continuing to investigate.
See Cauldron Room/Test logs and notes for more details.