The revelation of the unstable core within the sunken city weighed heavily on Amara and Kael as they prepared to dive once more into the abyss. The core, a vestige of the city's former glory, now posed a threat that could unravel the very fabric of their world.

"It's like a sleeping giant," Kael said, his voice steady despite the gravity of the situation. "We don't know when it might awaken, or the havoc it could wreak if it does."

Amara nodded, her resolve firm. "We have to find it, understand it, and ensure it can never harm anyone."

Their descent was a journey not just through water, but through time. The city's silent streets were a mausoleum to human ambition, and the core was its heart, still beating with a dangerous energy.

As they navigated the labyrinthine tunnels, the bioluminescent glow of the shell illuminated their path, casting an otherworldly light on the ruins. It was a beacon that had led them to the truth, and now it guided them to the core.

The core itself was encased in a chamber, a spherical room with walls that shimmered with a metallic sheen. The energy within pulsed like a living thing, a rhythmic throb that resonated with the melody of the shell.

"It's beautiful," Amara whispered, her voice filled with a mix of awe and fear. "But it's like a bomb waiting to explode."

Kael's eyes were fixed on the core, his mind working through the possibilities. "We need to find a way to neutralize it, to render it harmless."

They studied the chamber, the inscriptions on the walls providing clues to the core's operation. It was a puzzle that required both their knowledge and intuition to solve.

Hours passed as they worked, the danger of their task never far from their minds. The core was a remnant of a past that had been both brilliant and catastrophic, and they were determined to prevent it from defining the future.

With careful manipulation and a deep understanding of the technology, they managed to stabilize the core, ensuring it would no longer pose a threat. It was a victory, small but significant, in the shadow of the sunken city.

As they made their way back to the surface, the melody of the shell was a quiet accompaniment to their thoughts. They had faced the hidden danger and emerged triumphant, but their journey was far from over.