The Secret of the Underground Storage location (Kalaja)

Clue

"No matter how you look at it, that underground storage is suspicious." The coworker on the same night shift whispered quietly. "On an ambitious night, 5 or 6 of them go down there quietly and then return a few hours later completely exhausted." As they spoke, they kept glancing toward the cathedral, worried that someone would be listening.

Story

"I heard people come back super tired after a few hours." He said to me as he glanced at the cathedral in case someone might be listening. "Aren't you curious what happens there? Do you want to go see later at night?" I smiled at him. "Sure. I think it will be a fun night."