Stand at the edge for ten minutes and the sound resolves: a low swell with a period of eight seconds, a hiss of breaking over the sand bar, and beneath both a slow undertow that never stops moving.
Fetch a beach on a stormy afternoon and you can read the weather that arrived hours earlier — fetch length becomes wave height, wave period becomes swell length. The beach is a logbook.
The continental shelf ends in a slope. Water arriving over it converts its horizontal push into a downward slide — up to four kilometres of vertical drop in a few kilometres of travel, and the only place it surfaces is in the roar of surf.
Below two thousand metres there is no light, no wave and no year. A signal sent into that column takes three hours to return. The abyss keeps time in a way no other place on Earth does.
Four hydrophones, one kilometre apart, recording the same stretch of coast since March. The dots below are not decoration — each one is a contact, heard and classified.