India, Oct. 8 -- From the deck of a ship, the Sargasso Sea looks clean. The water is a deep, calm blue, and the sea lies mostly in international waters in the North Atlantic. But in August 2019, actress Shailene Woodley joined a Greenpeace research trip there, and the nets told a different story. The team discovered "extreme" microplastic pollution there, according to a press release from Greenpeace International. One water sample alone contained 1,298 tiny bits of plastic.

Woodley was there as a Greenpeace ambassador for the oceans. The ship, the Esperanza, was part of a wider Pole to Pole expedition to draw attention to threats to the ocean. Scientists aboard used fine nets called manta trawls that skim the sea surface to capture small...