Blue whales are the largest and heaviest animals that have ever lived on Earth. These mammals can weigh more than 200 tons and grow over 33 metres long. They are named after their blue and grey-ish colouring on their upper side. Their underside is usually anywhere from white to yellowish. Blue whales are baleen whales, using their baleen plates to filter plankton from water. During summertime, a blue whale feeds on up to 3.5 tons of plankton each day.