Pretty sure this is brand new.
Sean, always careful when he dove, prepped as before at the USS Boston memorial.
Nothing and no one.
He put on gear not usually carried on a free dive: a small tank with a thirty minute air supply, a headlamp and a backup snapped to his belt, night vision goggles, an Argonaut knife strapped to his calf, and a multi purpose camera on a chest harness.
One last equipment and harness check and he slid into the water.
At the cave mouth he checked his watch and rechecked his gear and harness.
Okay, Sean. Let’s go exploring.
He surfaced so slowly and clean it seemed the ocean hesitated letting him go, as if fearful of losing one of its own. No wake, no ripple, less sound than that caused by the tunnel’s natural pulsing with the waves and tides.
He waited, only the crown of his head down to his eyes above the waterline and at the edge where the tunnel opened to the cave floor. He rotated slowly to go unnoticed by anything living be in the cave.
Nothing save the bioluminescence of the algae lining the walls. Did this cave fill at high tide? Water seeks its own level, but to fill the air in the cave must escape.
He focused and rotated again, this time seeking areas where algae didn’t cling to the walls.
Ah! There and there. And two more over there.
His hands shifted from maintaining depth to slightly lifting until his ears surfaced.
Again, nothing. Only the sound natural to a underground, underwater cave.
The water rippled softly.
He put his hands on the edge of the floor.
The passive sonar thing from before?
No, not the same.