Forgotten structures are artificial constructions created by humans that have lost their original, defined purpose. Their common characteristic is their gradual or sudden disappearance from collective memory. The materials used in these structures depend on the technological capabilities of the era in which they were built.

Based on the degree of forgetting, they can be classified into three categories:

1. Partially forgotten: The accessibility of the memory diminishes but remains retrievable.

2. Subconsciously latent: Memories reside in the subconscious and can only be recalled under special circumstances.

3. Cosmic: Accessible through ancient, cosmic memories.