A flooded subterranean megastructure, ancient stone corridors half-submerged in

A flooded subterranean megastructure, ancient stone corridors half-submerged in black water, bioluminescent algae pulsing along the walls like a heartbeat. A masked diver in scarred tactical gear, lit by a flickering wrist-mounted flare, is pulled downward through collapsing flooded chambers by the current of a colossal eyeless leviathan moving beneath the floor grates. The camera plunges after the diver as water rushes through shattering stone archways, air bubbles and debris spiraling past the lens. The diver grabs a broken pillar to stop herself, the current ripping at her body, then releases and freefalls through a vertical shaft, twisting to dodge the leviathan's translucent tendrils whipping upward through the darkness. Quick shaky frames catch her igniting a harpoon gun mid-fall, the flare illuminating the creature's massive jaw opening below. In the final movement, the camera spirals downward around her as she fires the harpoon into the creature's mouth, the bioluminescence erupts blinding white, and the camera bursts through the surface of a vast underground ocean, the diver silhouetted against the glow of a thousand pulsing organisms on a cathedral-sized cavern ceiling. 15 seconds..