The cave was deeper than they had imagined.
The glow from the luminous mushrooms could only illuminate a few meters around them. Beyond that was pitch-black, with only the sound of flowing water echoing beneath their feet.
"This cave is really deep." Gon walked at the front, holding up a luminous mushroom. "We've been walking for almost half an hour, and we still haven't reached the end."
Killua stood on his skateboard, pushing off the slick cave wall to accelerate and overtake Gon.
"A tributary of the underground river," he said. "Listen. The sound of the water is getting louder. There should be an exit ahead."
Kurapika walked in the middle, his chains slowly circling around him as he kept watch over their surroundings. The Weather Shell sat in his coat pocket, trembling faintly ever since they entered the cave.
"Something's wrong." He suddenly stopped.
Everyone else stopped as well.
"What is it?" Leorio asked.
"Smell the air," Kurapika said. "There's something in it."
Everyone sniffed.
They had not noticed it at first, but now that they paid attention, there was indeed a faint, pungent smell in the air. It was like something rotting, mixed with a hint of sourness.
"It's drifting over from ahead." Genos's optical lenses flickered. "I detected trace amounts of corrosive gas in the air. The concentration is rising."
"Corrosive gas?" Leorio frowned. "Is it poisonous?"
"The current concentration is very low and harmless to humans," Genos said. "But the farther in we go, the higher it becomes."
They continued forward.
After another ten minutes or so, the changes became increasingly obvious.
The water in the tributary beneath their feet changed from pale blue to light green, as if someone had mixed in paint. Tiny bubbles began rising on the surface, and when they burst, wisps