Chapter 519: Exploration (4)

Around one month later, Billy was almost finishing the submarine. While he didn't have all the technical knowledge to replicate one from Earth, he still managed to make most of the things that he needed by creating magic tools that would do most of the job to keep them alive. There was a magic tool to make mana create breathable air at a slow rate since too much oxygen was also a problem. He wondered if the fact that magic that improved his brain made him recall those things.

Air contains about 21% oxygen. Breathing in higher oxygen concentrations can cause oxygen toxicity. Oxygen toxicity can affect all the body's organs but most often causes damage to the lungs, eyes, and brain. Most people recover from oxygen toxicity. But it's still a good idea to avoid high oxygen concentrations when possible.