Down in the Cave

It was quiet in the cave; Yun Meng didn't feel any abnormal vibe or a strange scent. So, she walked into the largest tunnel. As she moved forward, she felt the tunnel leading downward. About half an hour later, she entered another large round room.

She spent about three hours moving downward and made a few circles in the underground cave, but still didn't find anything unusual. At last, the road in front of her ended. It seemed to be a fault. In front of her was a cliff, and over ten meters down the cliff was an underground river, wide and deep.

Only after approaching the cliff did Yun Meng sensed a faint vibe that was strange and coming from the river. She squatted by the cliff and carefully looked down. She couldn't see colors; in her eyes, the river water which was supposed to be translucently gray was pitch dark.

Why was the water black? Was there some problem with the river? Was that why those underground gorillas ran out?