Inside of cauldrons and the Boundary

Now, multiple metal plates overlap to close the mouth of the device, but if they open, the inside is like the crater of a volcano. A fiery, lava-like substance swirls, burning fiercely as if it intends to swallow anyone who dares to look inside.

But what swirls within is neither lava nor flame.

It is another world. A distortion in the fabric of reality, a place where humans should not be. The other side of a gaping rift. The outside of the world's shell, a place that should never be seen, never entered.

The metal device he looked down upon was the one controlling and interacting with this "cauldron." A device to touch the otherworldly realm - the Boundary - that humans were never meant to reach.
- Blazblue: Calamity Trigger Part 1 Chapter 1