The invisible box, or ice cube, is the most ubiquitous metaphoric image referred to by the 16,000 suicidal people Matteson has worked with.

The "ice cube" is a metaphor for what the suicidal person is trapped in. It is a contextual trap they can't consciously identify, so is invisible to both the "trapped" suicidal person and those wanting to help them "get out" of the pain they are in.