Half a century ago, commercial divers on California’s Central Coast could make a killing from harvesting red urchins, an expensive delicacy. These days, they’re diving for another reason: to collect and destroy purple “zombie” urchins, which have been linked to the decline of kelp forests and are out-competing more valuable species—like their red cousins and abalone—in the race for food and, thus, survival. Grist follows divers as they journey through coastal ecosystems in search of “purps,” as they’re commonly called. There’s no guarantee the efforts will pay off yet: Experts and local environmental advocates are worried about the ripple effects these practices may set off.