While it has long been known that the ocean contains, in aggregate, large amounts of valuable minerals dissolved in it, extracting them has been expensive and difficult.
Attempts to extract minerals using ion exchange started shortly after World War 2. In the 2002, Japanese researchers at the Japan Atomic Energy Research Institute advanced a method using flexible plastic fibers braided into mats and impregnated with an absorbent chemical. The costs involved are around $1,200 per kg using this method, as opposed to the approximate $120 per kg from traditional mines.
Recently researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL) and Pacific Northwest National Laboratory (PNNL) have more than doubled the amount of uranium that can be extracted from seawater using refinements on the Japanese method. They've successfully brought the costs down to about $660 per kg.
Why all the effort? The world's oceans contain around 4.5 billion tons of uranium, enough fuel to power every nuclear plant on the planet for 6,500 years.