Salt. It might not seem particularly interesting, or have any historical or strategic importance. But the opposite is true. Salt has always been important to mankind, for preserving food as well as flavoring it. It helped ensure food was available out of season and allowed it to be transported over greater distances. Salt is the only rock that humans can eat, and it has been a valuable commodity throughout history. Roman legionaries were even partly paid in salt. The word "salary" (salarium in Latin) is derived from the word salt. Salt production sites and salt trade routes have always been of great strategic value. In the 19th century, salt became essential to the development of the chemical industry. High-purity salt was used to produce essential salt derivatives such as chlorine and soda. Nowadays, salt caverns are also used to store energy.