 
Carbonic acid H2CO3 is a weak acid that forms when carbon dioxide dissolves in water. It's harmful if swallowed, inhaled, or comes into contact with the skin or eyes. Carbonic acid is a key part of the global carbon cycle and biological systems that contain carbonate. However, carbonic acid quickly breaks down into water and carbon dioxide in the presence of water. In wastewater treatment and agriculture irrigation, carbonic acid is used to acidify the water. As human industrialization has increased the proportion of carbon dioxide in Earth's atmosphere, the proportion of carbon dioxide dissolved in sea and freshwater as carbonic acid is also expected to increase. This rise in dissolved acid is also expected to acidify those waters, generating a decrease in pH. It has been estimated that the increase in dissolved carbon dioxide has already caused the ocean's average surface pH to decrease by about 0.1 from pre-industrial levels. Carbonic acid is important in the transport of carbon dioxide in the blood. As carbon dioxide enters the blood, it combines with water to form carbonic acid, which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). Blood acidity is minimally affected by the released hydrogen ions because blood proteins, especially hemoglobin, are effective buffering agents. Carbonic acid plays a role geologically in the creation of caves and cave formations like stalactites and stalagmites. The largest and most common caves are those formed by dissolution of limestone or dolomite by the action of water that's rich in carbonic acid derived from recent rainfall. 
Significant amounts of carbonic acid under pressure exist in aqueous (water) solutions in planetary interiors, in particular in the cores of large icy satellites such as Ganymede, Callisto, and Titan, where water and carbon dioxide are present. Pure carbonic acid, being denser, is expected to have sunk under the ice layers and separate them from the rocky cores of these moons.
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