Carbon sequestration and carbon capture are processes to remove carbon dioxide (CO₂) from the atmosphere and store it. This is exactly what plants do when they convert CO₂ into other molecules like sugars and proteins. Carbon dioxide can also react with water and limestone (CaCO₃) to form calcium bicarbonate (Ca(HCO₃)₂) in an inorganic example of natural carbon sequestration. Generally, though, this term is used today to refer to man-made processes for removing CO₂ from the atmosphere or for capturing it before it gets released (like from power plants). A number of approaches to long-term carbon dioxide storage are being, or have been, tried, including pumping the gas deep underground into natural rock formations, scrubbing the CO₂ out from flue gas by reacting it with bases, or transforming the CO₂ into other useful molecules for making polymers, just to name a few.