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.

What’s going on here? 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.