Every year, tens of thousands of people worldwide volunteer for the Sisyphean chore of picking up trash from beaches. The largest effort is conducted every September by the Ocean Conservancy, which in 30 years of cleanups has collected 300 million pounds and more than 350 types of items.

New trash inevitably appears. Sometimes it takes only minutes. Nicholas Mallos, who directs the Ocean Conservancy’s Trash Free Seas program, has watched that in amazement.

“I have been on beaches in Hong Kong, Saint Helena in the South Atlantic, and Indonesia where you can watch plastics and debris in the barrel of each wave crash onto the beach. Literally, the trash starts getting replaced as soon as you pick it up,” he says.