AVARUA, Cook Islands — Hans Smit, CEO of Moana Minerals, placed a bucket on a table. It was full of what looked like irregular black golf balls. The balls were heavy but surprisingly fragile, shedding thin flakes at the touch of a hand.

Over millions of years, these polymetallic nodules, as they are called, have formed around grains of sand, shark teeth or sea shells at the bottom of the ocean. “We can film for kilometer after kilometer and the bottom is completely covered with them,” says Smit.

Read More