SCP-3000 is a colossal moray-eel-like serpentine entity residing in the deep waters of the Bay of Bengal, particularly the Ganges Fan, stretching roughly 600 to 900 kilometers in length. The compound it secretes, Y-909, is the substance the Foundation depends on as the core ingredient of its amnestics. For that value SCP-3000 is classified Thaumiel — judged beneficial to humanity.
The heart of the horror is the dissolution of the self. Personnel who observe it closely suffer a cognitohazard in which foreign memories seep in and the sense of self erodes. Its sheer scale and identity-eroding effect reveal how fragile the boundary of "I" is before a being that overwhelms the human scale.