what mysteries occur in the hot afternoon sun, and how are they explained?

In the beginning of this novel, the setting is shrouded in mystery. It is the very exotic natural setting that these British boys have likely never experienced,

Strange things happened at midday Sometimes land loomed where there was no land and flicked out like a bubble as the children watched. Piggy discounted all this learnedly as a “mirage”; and since no boy could reach even the reef over the stretch of water where the snapping sharks waited, they grew accustomed to these mysteries and ignored them, just as they ignored the miraculous, throbbing stars. At midday the illusions merged into the sky and there the sun gazed down like an angry eye.