The swelter of salt
Haneul checking the machinery that would absorb 500 atmospheres of pressure 5 kilometres below the ocean's surface where air cavities in living things are crushed and proteins fold in on themselves. The piezolytes printed into the vessel's carbon skin would help stabilize the thing down there, but make it reek of thick wet fish. The marine engineering unit calls it Jijneun.
Trawling the trench for manganese nodules, kicking up iron ores the size of walnuts. Thermal vents superheating the water without boiling it from the pressure. Extremophile life forms metabolizing heat gradients at vent lips lined with the richest ores. Extremophiles, metals, and mining equipment that smells like dead fish – that's all that's down there.
Not even dead matter – those bits of captured sun glinting slower and slower down the ocean’s trophic cascade until the oppressively heavy nothing of the bottom stops the few carbon molecules left. Just the howling of the vessel as it scrapes the bottom.