The Boundary is filled with a lava-like substance

"Damn..." Ragna cursed under his breath, standing still in the overflowing light. Without taking a step forward, he peered into the cauldron. Beyond the giant hole was a crucible of light and heat. It looked like molten lava or molten glass, swirling in a viscous mass.

That was the Boundary. And at the top of the cauldron, illuminated by the otherworldly light from below, was a magnificent decoration that looked like the hilt of a sword thrust into that Boundary.
- Blazblue: Calamity Trigger Part 1 Chapter 5