Cause

The underground tunnel was basically a hollow pillar of stone that shot up from the underground world to the surface. 

Their size could reach as far as two hundred meters in radius. 

However, the hole this time was only about forty meters in radius. It was used to create several elevators that could bring a lot of things to the underground world.

But the truly shocking sight was not the fact that the hole was huge or had majestic equipment. It was the fact that the tunnel was actually being filled with rubble. 

"This is…" Clovis gasped. "The tunnel has collapsed?"

"No. It's impossible for the tunnel to collapse. With all the tunnels in the world, we have no record of them collapsing." Kanaria shook her head. 

Jay raised his eyebrows. "Artificial?"

Erwin nodded his head with a grim expression. "Exactly so. That's what we currently believe to be the case."