Given the task definition and input, reply with output. In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: Every year gray whales swim a 10,000 miles round-trip, from the cold waters near Alaska, down the coast of North America to the warm waters near Mexico, and back again. It is one of the longest migrations of any mammal. All day, all summer long, in the cold waters near Alaska, gray whales eat and eat. Instead of teeth, gray whales have baleen, they are used to getting food from ocean water. They get big mouthfuls of muck from the ocean floor, then push that muck right back out through their baleen. The mucky water flows through, but tiny animals get trapped in the baleen and _ . When the water starts to freeze, the whales begin their long trip south. They swim night and day, without stopping to eat or rest. During the long journey, migrating whales may push their heads out of the water to see where they are. Two months later, the whales reach Mexico. Babies are born here in warm, shallow waters called lagoons. Ocean lagoons have no food for the adults, but the babies are safer here than in the open seas. All winter the babies grow big and strong. In spring first the dads and teenagers leave the lagoons, then the moms with babies. Migrating gray whales swim close to shore, especially moms with babies. The ice is melting up north, and it's time for the hungry whales to make the long journey back to their summer feeding grounds.
Answer: always swim under the ocean
During their long journey to Mexico, gray whales _ .