The first aerial survey of the Great Pacific Garbage Patch shows that the amount of debris swirling in the North Pacific has been “heavily underestimated,” the expedition group said.

On Monday, The Ocean Cleanup, a project founded in 2013 by then-18-year-old Dutch inventor Boyan Slat with the goal of ridding the world’s oceans of plastic, shared initial findings from its aerial expedition of the trash vortex between Hawaii and California.

Researchers documented more garbage at the edge of the gyre than they expected to see at its center, where debris is more concentrated, Slat said at a press conference at Moffett Airfield in Mountain View, California. In just 2 1/2 hours, he said, the crew observed more than 1,000 large floating objects.

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“Although, again, we still need to get a detailed analysis of the results, I think it’s really quite safe to say that it’s worse than we thought,” Slat said. “This underlines the urgency of why we need to clean it up and that we really need to take care of the plastic that’s already out there in the ocean, because all this big stuff, over the next few decades, will crumble down into those small microplastics.”

Understanding how much marine debris is out there will be essential to Slat’s ambitious plan to clean it up, the foundation said.