In contemporary debates on risk in modern societies, on reflexive modernity and a general crisis of knowledge, concepts and terms such as ignorance, non-knowledge or negative knowledge are used to denote that there can be knowledge about what is not known. Many of these terms are not only used with different meanings, sometimes antithetic to one another in their implications, but they often propose tree-like taxonomies without broaching the issue of the further connectivity of different types of unknowns between the limbs of the tree. In this article, an attempt is made to simplify and integrate different connotations in sociological usage of concepts that try to grasp the unknown and to outline the dynamic and recursive relations of these types of knowledge and the way they can change over time. This is illustrated with examples from large-scale ecological design projects.