Ben Lecomte and his team left Hawaii last month. They've now reached the center of the patch.
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The group has been towing a net and collecting micro-plastic.
"Right now, being in the vortex, in the core of it, we collected, at one time, 3,000 pieces of micro-plastic in 30 minutes," Lecomte said.
They say they've seen floating trash every day and witnessed how it's changing the ecosystem.
Lecomte hopes to reach San Francisco in late August.