辨认主题句：
Read the following paragraph and underline the topic sentence.

Our oceans are in more trouble than ever before due to the flow of plastic into the sea. Everything from plastic bottles to bags ends up in our oceans each year. Traveling on ocean currents, this plastic is now turning up in every corner of our planet. It is even being found in the deepest part of the ocean and trapped in Arctic ice. Our oceans are slowly turning into a plastic soup, and the effects on ocean life are devastating. Plastic pieces of all sizes choke the stomachs of creatures who mistake them for food, from tiny fish to whales. Plastic is now entering every level of the ocean food chain and is even ending up in the seafood on our plates.