The nitrogen cycle
The nitrogen cycle is one very important thing that happens greatly to our benefit.
When I look at the floor, I cannot fully imagine the processes that are happening right there under my feet. It amazes me when I see the atmosphere, which is filled with a gas nitrogen among other gases, but it will never be used by the plants that are feeding us. I see it as nature decided to make this resource completely useless and not reacting, leaving no chances to turn it into anything helpful until some kind of activation happens. Mother nature found the solution to this problem though.
Using the activation process I just mentioned, nature converts the useless gas into nitrates by making small organisms perform certain actions with it and change its state. It is really helpful to our ecosystem since without such actions, plants would dry out and will not be able to develop proteins inside their cells. This fact proves the uselessness of the most abundant elements if nothing else contributes to their work.
I would define the nitrogen cycle as a control system of our natural environment. It allows nature to balance between plenty in the atmosphere and the lack of nitrogen on the Earth. We can say that our entire ecosystem is based on the closed circle of the nitrogen movement through the atmosphere, soil and also the organisms. Once the plants get enough nitrates and use them, they turn back to the earth.
I think most of us are not conscious and usually forget that nature is doing a lot of hard work under the plants and we should acknowledge it. Most people think of plants only as the source of their food, forgetting how much effort the soil spends to let life flourish every year.