Courtesy of Arm & Hammer (http://www.armandhammer.com) "Baking Soda, alias sodium bicarbonate, is a naturally occurring substance that is found in all living things, where it helps regulate their pH balance. ARM & HAMMER® Baking Soda is made from soda ash, also known as sodium carbonate. To make ARM & HAMMER® Baking Soda, the soda ash is mined in the form of an ore called trona. The soda ash is then dissolved into a solution through which carbon dioxide is bubbled and sodium bicarbonate precipitates out, forming baking soda."

Short answer, the Solvay Process. Long answer - (from http://en.wikipedia.org/wiki/Solvay_process) The Solvay process produces sodium carbonate from salt (sodium chloride) and limestone (calcium carbonate). In neutral or basic solutions, sodium bicarbonate is less water-soluble than sodium chloride. When carbon dioxide passes through a concentrated solution of sodium chloride and ammonia, sodium bicarbonate precipitates according to the following chemical reaction: NaCl + CO2 + NH3 + H2O → NaHCO3 + NH4Cl The ammonia is necessary to buffer the solution at basic pH. Without it, a hydrochloric acid byproduct would render the solution acidic, and in acidic solutions, the reaction cannot proceed. In industrial practice, the reaction is carried out by passing concentrated brine through two towers. In the first, ammonia bubbles up through the brine and is absorbed by it. It the second, carbon dioxide bubbles up through the brine and precipitates sodium bicarbonate. Carbon dioxide for this step is produced by heating calcium carbonate: CaCO3 → CO2 + CaO The solid sodium bicarbonate is then filtered out and converted to sodium carbonate by heating it, recovering some carbon dioxide in the process: 2 NaHCO3 → Na2CO3 + H2O + CO2 Meanwhile, ammonia is recovered from the ammonium chloride byproduct by treating the ammonium chloride solution with the calcium oxide (a strong base) left over from carbon dioxide generation: 2 NH4Cl + CaO → 2 NH3 + CaCl2 + H2O The recovered carbon dioxide and ammonia are recycled back to the initial brine solution. When properly designed and operated, a Solvay plant can reclaim can reclaim almost all its ammonia and consumes only small amounts of additional ammonia to make up for losses. The only major inputs to the Solvay process are salt and limestone, and its only major byproduct is calcium chloride.