Carbonic acid is a compound of hydrogen, carbon, and oxygen. It is formed in small amounts when carbon dioxide dissolves in water. Although it is a weak acid, it can dissolve limestone, which leads to the formation of many features of this mineral, including stalagmites and stalactites.
Carbonic acid has been increasing in the oceans because of the increase in carbon dioxide in the atmosphere as a consequence of the massive burning of fossil fuels. The resulting ocean acidification can be very harmful to marine organisms by interfering with the growth of their calcium carbonate shells and making it difficult for them to reproduce.