The The Blue Whale (balaenoptera musculus) is generally considered the largest animal that ever lived. It can reach lengths of over 110 feet long and weights of over 200 tons.
It's hard to imagine what that is in real terms. 110 feet is over 1/3 the length of a football field. To give an even better conception of how big that is, I've included a photograph of the life-size blue whale model from the American Museum of Natural History in New York.
The Blue Whale is a carnivore. He hunts large shoals of tiny shrimp called krill. The whale will swim through the krill with his cavernous mouth open taking in as much water and krill as he can, then he closes his mouth and uses his tongue to push the water back out letting his feather-like teeth called baleen strain out the shrimp which he then swallows.
The throat of the Blue whale is pleated so it can expand like an accordion when he takes in water. The image above shows his throat about half expanded. For a long time, scientists thought the whales throat was always partially expanded because they only time anyone go to see the underside of the whale was when one beached itself or was harvested by whalers.
Compare the images below of a free swimming blue whale with his throat deflated and another image of the whale with his throat full of water and krill.