Arraia shoots back coldly, "I guess she just wanted a souvenir of the trip."
Estre clicks in disapproval. "Now, don't say that. You're talking about a person." As the three of you swim farther down slowly, the light from up above fades, and your natural phosphorescence takes over.
"Tephra shouldn't have brought it down," says Arraia. "Humans don't belong here. Their bodies, their minds can't take it, and it's not fair to them."
This new human must be going through the same process that you know happens to humans who are around merfolk down here. Their bodies fill with salt water and strengthen against the crushing cold, but getting used to the sensations takes a while. After a time, the merfolk's immortality even rubs off on them, and they stop aging.
Estre wriggles uncomfortably. "This new human, it chose to come down here itself, knowing it would have to stay," he says. "Do you really think it shouldn't be allowed to control something like that in its own life?"