Perhaps they migrated there through evolutionary pressure, and/or there's phosphorescence that we didn't see because of our giant camera light.
Well if they weren't blind they definitely are now with that probe shining bright lights in their eyes. Assuming they were adapted for really low levels of light, they might just be vestigial.
That bothered me too. Perhaps there's an underground tunnel connecting that area with the ocean?
IMHO that almost has to be the case. There is no reason for anything down there to have eyes.