For decades, entrepreneurs have tried to strike it rich by gathering up ugly potato-size rocks that carpet the global seabed. Known as manganese nodules, the rocks are plentiful in nickel, copper and cobalt, as well as manganese and other elements, but lie miles down in inky darkness. Building giant machines to vacuum them up, despite much study and investment, has never proved to be economic.

Now, the frustrated visionaries are talking excitedly about the possibility of belated success, and perhaps even profits.

The nodules turn out to contain so-called rare-earth minerals  elements that have wide commercial and military application but have hit a production roadblock. China, which controls some 95 percent of the world’s supply, had blocked shipments, sounding political alarms around the globe and a rush for alternatives. China ended its embargo late last month, but the hunt for other options continues.

So are seabed miners smiling at last?

“People are quite intrigued,” said James R. Hein, a geologist with the United State Geological Survey who specializes in seabed minerals. Depending on China’s behavior and the global reaction, he said, “rare earths may be the driving force in the near future.”