In the early portion of the 20th century, chemists hoped they could develop a process to recover pure gold from large quantities of seawater, making themselves and their countries rich. German chemist Fritz Haber, famous co-inventor of the Haber-Bosch process, spent a portion of his career attempting to extract gold from the sea to pay for Germany’s post-WWI debt.

Seawater contains, on average, 0.1 to 2 mg/ton of gold, depending on location. The problem lies in getting the gold separated from the water. It would take decades to process enough water to filter out the gold, considering the current capacities of the technology. The value of the gold extracted would be very unlikely to equal the cost of such a venture. From the perspective of chemists in the early 20th century, it must have seemed like an even larger quantity.

Nevertheless, the lure of gold is great, and throughout history people have spent their lives trying to get it, whether through stealing it, transmuting it from lead, or extracting it from seawater. Of course, these chemists failed in their endeavor. Successfully extracting the gold would require a series of massive centrifuges, so costly that the gold extracted would never be able to pay for their construction.

It turns out that the only mineral that can be extracted from the ocean profitably is salt. A cubic mile of seawater's worth of salt, it turns out, is enough to supply the entire world for upwards of a year — 128 million tons. It is extracted by collecting it in areas where seawater evaporates, leaving salt deposits behind. The ancient Chinese did this as long as 3,000 years ago. Today, most salt is obtained from brine wells and salt domes, which were created millions of years ago when ancient seas evaporated.

Another metal that has been successfully isolated in significant quantities from seawater is manganese. In some areas of the world, the ocean bottom is covered in many trillions of manganese nodules, the dust of which is dissolved in the sea water. In fact, as much as 1/1000th of seawater, by weight, is manganese.

There is so much manganese in the ocean that it is the primary source for many large countries, such as the United States. The extraction process is simple. It may not be gold, but it does come in handy.