Atlas Obscura:

In the coverage at the time, almost all the media outlets (including Sports Illustrated and World Press Photo) described the image as a physical manifestation of the breaking of the sound barrier. Gay seemed to believe that, telling one interviewer, “I clicked the same time I heard the boom, and I knew I had it.” Other coverage described the cone as a result of the Prandtl-Glauert Singularity, a phenomenon predicting that aerodynamic forces would approach infinity as aircraft neared the sound barrier.

It turns out neither of these are correct. Instead, Gay had captured an effect known as flow-induced vaporization that sometimes forms around objects flying at high speeds in the right environmental conditions.

It’s an amazing photograph that has been around for years and almost always incorrectly described as a “F/A-18 Hornet breaking the sound barrier”. The truth is a lot more complicated.