12 Hours to 50 Days

 After that, Yu Na led us back to her training center.

I thought we'd be going to one of the rooms, but she brought us deep underground instead. It took us about ten minutes just to walk down the stairs, spiraling downward. When we reached the landing, the space before us made everyone gasp in awe.

"That's... big."

"It's like a skyscraper fitted inside the underground!"

"Fatima, what's a skyscraper?"

"A skyscraper is a tall building, mocking gods by reaching for the heavens above."

As the girls were chatting, Yu Na walked to the side of the building before opening something that looked like a chute.

"You can drop the essence crystals for the fuel here. It should be consumed automatically as soon as you enter and close the door."

As she explained, she moved to the dial beside the door. There, a series of numbers were shown, from 0 up to a thousand. Yu Na changed the value from the 10 it had before to the 100 that I asked for.