Rock salt is a mineral, halite. It occurs in massive underground deposits on all continents. In its pure form it is sodium chloride, a crystalline substance composed of equal numbers of sodium and chlorine atoms.

In the halite crystal, electrical attraction holds positively charged sodium ions and negatively charged chlorine ions together to form a three-dimensional structure that has alternating sodium and chlorine ions in all directions. The sizes of the sodium and chlorine ions form a rectangular structure, which is reflected in tiny rectangular, often cubic particles (officially rectangular parallelepipeds) of salt crystals.

Advanced life forms could not exist without it. Salt is involved in regulating the fluid balance of cells, and its sodium ions are essential for the functioning of neurons that control muscles such as the heart.

Salt and temperature drive the circulation of the world ocean. Density changes due to seasonal and geographical variations in evaporation and precipitation, freezing and thawing, warming and cooling power the thermohaline conveyor. This current is a three-dimensional loop which circulates deep and surface water through the Atlantic, Pacific and Indian oceans.

Salt’s use as a preservative dates from as long as 12,000 years ago. Chinese records date back 5,000 years. By then Egyptians had built storehouses of salt-preserved food to sustain them through the unpredictability of drought and famine. Salt was in use everywhere in Europe 3,000 years ago.

Phoenicians from the region that today is Lebanon were great sailors who established trading ports throughout the Mediterranean by 3,500 years ago.

One important locale was the island of Sicily, where masses of tuna swim by in their annual migration from the cold Atlantic Ocean. The savvy Phoenicians built evaporative salt ponds on an industrial scale to preserve the tuna catch and trade it throughout the Mediterranean.

From salt ponds they built all around the Mediterranean and mines in North Africa, they became the first superpower, but were overcome after 1,200 years by the Romans.

In ancient Greece a slave could be said to be “not worth his salt,” referring to his value in salt currency.

Rome ruled the Mediterranean following its defeat of Carthage, a Phoenician city in Sicily, in 252 B.C. Roman soldiers’ salary (from Latin: salarium, salt) was often bags of salt.

Venice, founded by refugees in northern Italy in the fifth century A.D., soon discovered the climate was perfect for evaporative salt ponds. The Venetians ran a lucrative business based on the Adriatic salt trade for hundreds of years, and the city is a grand center of art and architecture today as a result.

In the 13th century the northern Mediterranean climate became wetter, and the Venetian navy seized salt ponds in drier Mediterranean ports. From Constantinople, Venetians established trade routes to the exotic spices of the East.

They became even richer as they traded the spices to Europe, which sparked a culinary revolution and began a search for new trade routes to the lands of spices.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.