In chapter four the daily rhythm of life is set, as they have been on the island for a while. The morning is serene and pleasant and the evening is frightening. The day is a combination of both. The boys see odd physical things taking place on the island. However, they do not know what to make of this. Piggy, the most rational of the boys, believes that all these things are mirages - the way the sun hits the water. Here is the text:
Strange things happened at midday. The glittering sea rose up, moved apart in planes of blatant impossibility; the coral reef and the few stunted palms that clung to the more elevated parts would float up into the sky, would quiver, be plucked apart, run like raindrops on a wire or be repeated as in an odd succession of mirrors. 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.
As the story progresses, there is no real explanation of what is going on. However, the important point here is that the tone begins to be ominous. The reader expects a turn for the worse, which does come.
As for what this could be, Piggy probably offers the best explanation. Water and light can produce odd sights. Moreover, when you consider how scared and young the boys are, all of this is expected.
They didn't see what was right in front of them and they just continued with their lives, just as if they would've ignored something as preciouse as our gleaming, beautiful stars that, at night, are in our faces and can be ignored, but imagine our world without our stars, we wouldn't be the same, just as if we ignored what we have done most our lives and forget about it and continue on our way. They have just made a very horrible decision.