Water hardness

To understand this, we have to take a look at the water cycle. About 70% of the earth's surface is covered by water. This water evaporates in sunlight and water vapour is formed. This is lighter than air, therefore rises upwards and collects in the atmosphere. 

If the water drops are large enough, they fall back to the earth in the form of rain, snow or hail. 

The water carbonated (from air and soil) seeps into the soil and hits calcareous rock under the surface. 

The carbonic acid and lime react to form calcium hydrogen carbonate (Ca(HCO³)², colloquially lime).