Secret

Secrets are... light. Not emotionally, psychologically, but physically they have no weight at all, no matter how big they get.
That's why keeping secrets is an active task.
You need to be there, pushing them back under the black waters before the brownian motion of life jitters them to the surface, staring you silently in the face with their blank, dead, sightless eyes.
Dead men tell no tales, they said.
Dead men sing like canaries, because they lack both ability and reason to keep their secrets any more. Every misstep, foible, brutality and act of evil, big and small, surfaces sooner or later. The dead don't care. It's the living that run around pushing the tales back, carefully handcrafting that unnatural artefact of groupthink called truth.
Secrets stain the water, glow in the dark, whisper in the shadows, their odor faint but pervasive.
They are unmissable, unforgettable, immortal.