The Blue Whale is the largest creature ever to have lived on earth. Their tongues alone can weigh as much as an elephant. Their hearts, as much as a car. It is estimated that only10,000-25,000 blue whales now swim the world's oceans. Amazingly, however, this giant of the ocean feeds on some of the smallest marine life - tiny shrimplike animals called krill. A single adult blue whale can consume 3,6000kg of krill a day. They mainly catch their food by diving, and descend to depths of approximately 500m.