Researchers have identified the cause of a persistent optical illusion near Australia’s Lady Elliot Island, where visitors frequently report seeing a mysterious third lighthouse beam that does not exist. The findings, published in the journal Open Mind, explain how the human brain misinterprets light sources in low-visibility environments. Lady Elliot Island currently operates one lighthouse, which replaced an original structure built in 1873 that was decommissioned in 1995. Despite the presence of only one active light, tourists have long reported a phantom beam appearing on the horizon. This phenomenon is well-documented, with similar reports dating back to 1937 near the coast of Brittany, France. To investigate, researchers studied the phenomenon on-site and recreated it using virtual reality (VR) simulations. The experiments confirmed that the illusion occurs when an observer stands at least 70 meters away from the lighthouse. When the observer faces the sea with the lighthouse behind them, the rotating beam can appear to originate from the horizon in front of them, even when the observer is aware of the actual light source’s location. The study suggests that the brain functions as a “prediction system,” constantly reconciling sensory input with internal expectations. In total darkness, the brain lacks sufficient visual cues to determine the distance and scale of objects. When faced with a rotating beam, the brain may misinterpret the distant, fast-moving light as a closer, slower-moving object, leading to the perception that the source is in front of the viewer. This mechanism is similar to other well-known visual illusions, such as the “crater illusion,” where a concave depression is perceived as a convex hill because the brain assumes light is coming from above. Researchers noted that while this predictive mechanism usually helps humans interpret the world accurately, it can lead to errors when environmental cues are ambiguous. The study concludes that these illusions provide valuable insight into the cognitive processes the brain uses to construct our perception of reality.