The visual puzzle was posted by @Benowine on 16 February and swiftly attracted thousands of responses.
Among those offering their answers were acclaimed French football writer Philippe Auclair, retired Olympic swimmer Sharron Davies, and radio comedian and author Mitch Benn.
Some claimed to see only “528” in the middle of the spiral while others saw a longer sequence of “45283”.
In both instances, users expressed anxiety about what this might reveal about the state of their eyesight.
Others joked that they saw only the word “KILL” or speculated that the figure revealed the total number of parties prime minister Boris Johnson had hosted at Downing Street during lockdown.
In fact, the real answer is even longer at seven digits – 3452839.
The illusion, somewhere between a psychedelic magic eye poster and the notorious “what colour is the dress?” controversy of 2015, reportedly works by playing on contrast sensitivity, an aspect of ocular function that allows us to distinguish objects from their backgrounds.
A higher degree of contrast sensitivity is beneficial in low light conditions or in mist and fog when it is more difficult to perceive the difference between an object in the foreground and what lies behind it.
The puzzle plays a similar trick on the human eye by placing the swirls and numbers on different levels, meaning that those with strong contrast sensitivity are better able to pick out all seven digits.
Those in whom the quality is less pronounced may only be able to identify a shorter sequence.