This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: There are two lighthouses on Lady Elliot Island, at the southernmost tip of Australia's Great Barrier Reef. The first was built in 1873, but was decommissioned in 1995 when the second took over the job of warning ships off the reef.
Visitors to the island, however, regularly report seeing signs of a third lighthouse. At night the beams of a nonexistent phantom lighthouse are seen far off, shining on the horizon. This "phantom lighthouse illusion" is so well known it even gets a mention in the island's guide for tourists.
It's not unique to Lady Elliot Island, either: the earliest known report of this phenomenon was published in 1937, and it documented the illusion off the coast of Brittany in France. One of us traveled to Lady Elliot Island to find out, then we recreated the illusion in a virtual reality environment to make sure we understood what was happening. Like many illusions, this one seems to happen when our brains add our expectations to evidence from our senses and arrive at the wrong result.
To start, at night, stand at least 70 meters (230 feet) away from the lighthouse between the lighthouse and the sea. Look up at the lighthouse and you'll clearly see a rotating beam of light shining from where you'd expect. Now, turn around with your back to the lighthouse and look out to sea.
You know the light is coming from behind you but you'll be convinced what you're looking at is the light emanating from the horizon. It won't matter that you know this is wrong. No matter how many times you turn around to check the source, the phantom lighthouse is still there when you gaze again at the horizon.
The video below—from our virtual reality recreation—gives some sense of what the experience is like. In reality, the beam is generated by a lighthouse located 300m (980 feet) behind the observer. The video makes it appear as if the beam is generated by a moving source that lies in front of the observer.
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