Which layer of the atmosphere contains the ozone layer?

The stratosphere, and ozone is what makes it a distinct layer at all. Ozone absorbs ultraviolet and warms the air around it, so the stratosphere gets warmer with altitude - the inverse of the troposphere below - and that temperature inversion suppresses vertical mixing, which is why it is stable and why airliners cruise in the bottom of it.

The hole over Antarctica was found in 1985 by a British Antarctic Survey team who had been measuring ozone from the same station for decades. Satellite data had recorded the same collapse and the processing software had discarded the values as instrument errors because they were too low to be believable - the numbers were in the archive and nobody had looked.

What followed is the most successful environmental treaty ever agreed. The Montreal Protocol in 1987 phased out CFCs, every country in the world ratified it, and the layer is now measurably recovering with full repair projected for around 2066. It also had an unintended climate benefit: CFCs are potent greenhouse gases, and banning them avoided a substantial amount of warming as a side effect of fixing a different problem.