If it was not for the water, then all you could hear would be the echoes of Jack and Arthur's footsteps. Niles was walking without making a sound, not like either of them could hear it. They could only hear the beating of their heart, giving the impression that it was going to jump out of their mouth. They tiptoed through the cavern, hoping to not catch the attention of any beast residing here. Niles trailed behind by well over a dozen meters, they could not see him anymore.
Another growl reverberated through the water of the cave, and the vibrations of the water pounded on the force field around the two scared men. They paused, and after a whole minute in silence trying to discern if whatever monster was responsible for that horrible sound took notice of their intrusion. They began creeping through the cave again, and another growl hammered the force field again, but this time their eardrums also got pounded by the loud sound. They were closer to the beast.
They walked for almost a kilometre, or something like that, Arthur lost track. After walking a kilometre, there appeared shinning rocks embedded on the walls of the cave, lighting the way forward a pale shade of blue. Or more like lighting the way downwards, there was now a steep descent ahead of them. The creature bellowed again, and this time some water forced their way through the air bubble and their ears bled, temporarily deafening them. And despite the pain, they kept walking. It would not take long for their ears to recuperate.
All of a sudden the water in the cave just 'stopped', as if there was a barrier that did not allow the lake water to go through to the other side. Jack put away the necklace and signalled Arthur to do the same, now that they were surrounded by air and could fight like normal the necklace would just be a hindrance.
A few more minutes went by, and they finally reached the end of the cave. There was a large opening that went down about a hundred meters. It was rocky, and the celling was not made of stone, bur rather made of the same shining rock that lighted their way here. Jack and Arthur finally saw the perpetrator whose roars perforated their eardrums. Before seeing what the monster was, it growled again.
This time that water was not there to dampen the sound and that they were close to the source, the loud roar almost made the two fall all the way down right in front of the monster. The growl completely deafened them with a lot of pain accompanying the silence. They felt lightheaded, almost losing consciousness. They would have fell down if it was not for Niles holding them back. Jack and Arthur needed a couple of seconds to clear their head, although neither of them could still hear nothing. On the other hand, Niles seemed unfazed. Seeing that they were, in a way, ok, Niles held both of them up with his arms and jumped down.
Jack screamed at Niles, "What the hell are you doing!?" But instead of hearing his own voice, Jack heard nothing. There was no need to keep up the stealth act if Niles was going to straight up just jump down where the monster was. After what felt like an eternity, Niles landed on the ground, bending his knees to cushion the fall. Without even standing back up, he threw his two pupils on the ground right in front of the creature which looked startled by the intruders.
Now Jack and Arthur could finally identify the beast, and it was worse than they expected. It was a Hydra, and by its small size, a baby one. For Hydras, small meant at least five meters on each axis.
A baby Hydra was a low rank C, way stronger than that Cetus they struggled with. The Hydra roared one more time, before pouncing on them.