At the beginning of the 20th century, chemists hoped they could develop a process to get pure gold from a large amount of seawater and enrich themselves and their country. The German chemist Fritz Haber, who is a partner in the discovery of the Haber-Bosch process, spent part of his time researching to extract gold from seawater in order to repay Germany's second postwar debt.
The average sea water contains 0.1 to 2 mg per ton, depending on the location of the seawater. The problem lies in how to separate gold from sea water. It will take decades to process enough water and filter out the gold if we consider the current technology capacity. The value of extracted gold will not be at all worth the cost of the process.
Nevertheless, the temptation of gold is very large and throughout history humans spend time to get gold, either by stealing, trying to turn lead into gold, or extract from sea water. Of course, the chemists failed. The successful process of extracting gold will require a complex set of machines so that the gold obtained from the extraction will never be able to pay its extraction costs.
It is known that the only mineral that can be extracted from the oceans and the beneficial result is only salt. The salt extracted from one cubic mile of seawater is enough to supply the whole world for a year - about 128 tons. The salt is extracted by collecting seawater in an area that can evaporate it so that the sea water will evaporate and leave its salt. The Chinese people in the past have done this since 3,000 years ago. Today, most of the salt is obtained from salt wells and salt domes, created millions of years ago when ancient oceans were evaporated.
Other metals that have been successfully isolated from seawater in significant quantities are manganese. In some regions of the world, the ocean floor is covered by trillions of manganese nodules, grains that will be dissolved in seawater. In fact a millionth of the weight of sea water is manganese..
So much manganese in the ocean that the sea becomes the main source of manganese for some major countries such as the United States. The extraction process is simple. Perhaps the manganese is not gold, but it is definitely useful.