The antonyms for the word "were visible" could be words such as hidden, obscure, invisible, camouflaged, and imperceptible. When something is not visible, it means that it is hard or impossible to see, making it difficult for someone to notice or identify it. An object, person, or thing that is hidden or camouflaged may be intentionally obscured to avoid detection, often for protection or survival purposes. Alternatively, something may simply be imperceptible due to its small size or position, making it difficult to see without careful attention. Regardless of the reason, antonyms for visibility convey the idea that something is not easily noticeable or apparent to the senses.