We’ve made a mess of things. Humans have dumped some 250,000 tons of plastic debris into the world’s seas. Mostly floating as tiny, toxic particles, the nautical litter is killing animals and destroying delicate ecosystems. “It’s everything you can imagine made of plastic,” says the lead author of the study released yesterday. “It’s like Walmart or Target set afloat.” Based on data collected on expeditions made around the globe, the shocking figures may turn out to be conservative.