The question is a bit tricky! The ozone layer isn't *caused* by something, it's a natural phenomenon that occurs due to a specific process involving oxygen and ultraviolet radiation from the sun.

Here's how it works:

* Sunlight's UV radiation splits oxygen molecules (O2) into individual oxygen atoms (O).

* These individual oxygen atoms then combine with other oxygen molecules, forming ozone (O3).

So, the principle cause of the ozone layer is the interaction of oxygen with ultraviolet radiation from the sun.

It's important to remember that the ozone layer is a delicate balance. While it naturally forms, human activity, especially the release of ozone-depleting substances like chlorofluorocarbons (CFCs), has significantly thinned the ozone layer in the past. This thinning weakens the Earth's protection from harmful UV radiation.