When Acht opened his eyes again, he found himself standing in an open space that didn't exceed 2 meters in width. It was made out of normal stone with nothing there behind him.
So, he looked ahead of him. What he saw was a passage between two very big walls that seemingly exceeded to infinity from both angles. They were also made out of stone and were about 6 meters away from each other.
The first thing he noticed that was weird about these walls is that they had a huge amount of soul force embedded into them and ran through each part of the walls, seemingly like an endless stream of energy.
The second thing and it was the most important one is that they weren't motionless as all solid objects should be. They were somehow wiggling and moving at a very very slow pace. They were also barely noticeable to anyone. But, this discovery was more than enough for Acht to know something.