Ozone has the chemical formula O3, and it is a gas that is naturally present in the Earth’s atmosphere. Most of the ozone in the atmosphere exists as part of a layer that sits a few miles above the ground. This is the ozone layer, and it serves to protect the Earth from a significant fraction of the potentially harmful UV light from the Sun. Depletion of the ozone layer means that more harmful UV light reaches the Earth’s surface. Ozone is also a greenhouse gas, which means that it also plays an important role in controlling the Earth’s climate.