To make it sound more fantastical, there was a magic array outside the cave. It had to absorb Moon Essence during a full moon to open the cave.
The two put on hard hats with a light hanging from it. Otherwise, with their very human vision, they'd probably knock their brains out in the pitch-black cave.
"Did you bring magical phosphor?" Chen Zhao asked.
"Yes," Pierce said. "As a wizard, I have to have some on me."
"Sprinkle some in the front," Chen Zhao said.
"Why?"
"Just do it. Stop asking so many questions."
There were mayflies all over the place. They'd come from Hell and were trapped here by magic. They weren't lethal, but if they latched onto someone, they could cast some spells. For example, they could cause insufferable itchiness or pain somewhere.