Salt

The ocean is vast, and all (or nearly all) of the chemical elements are present dissolved in the water, so the ocean must contain huge quantities of important and valuable minerals. This idea has led to many schemes to extract one or another of the substances needed or wanted by mankind. In fact there are only a few rather mundane substances that can be extracted at a sufferable cost. The extraction of common salt has been of crucial importance for some peoples in the past, and continues to be economically favorable today. The extraction of water makes it possible for many to live where it would otherwise be impossible.

Consider the Maya. The Classical Maya civilization flourished some 1200 years ago in Central America in what is now Guatemala and Belize, and the adjacent parts of Mexico and Honduras. The evocative remains of their great cities such as Copán and Tikal, their extraordinary stone carvings and other objects, their elaborate and bizarre system of writing, and their detailed and accurate calendar with its implied care in astronomical observations have caught the imagination of many people since the first reports by John Stevens, who discovered Copán and other cities in 1841. This remarkable civilization existed in a hot and humid tropical environment, a situation that imposes certain constraints. With an estimated population of 5 million and little wild animal food available, the Mayans turned much of what now looks like unbroken tropical forest into agricultural fields. With no draft animals, they must have worked hard to tend these fields. The population of Tikal (near the middle of the region) is estimated to have been at least 45 000, a population not reached by the city of London, England, until centuries later.