A blue whale can be 30 metres long - that is the length of three school buses parked nose to tail. Its tongue alone weighs as much as a baby elephant. Its heart is the size of a small car, and its biggest blood vessel is wide enough for a child to crawl through.
Blue whales eat tiny shrimp-like animals called krill. Each krill is about the size of your fingernail. The whale swims with its enormous mouth open, takes in a whole swimming pool's worth of water, and squeezes it back out through a comb-like filter in its mouth. The krill stay behind. It is the world's most enormous animal eating one of the world's smallest.
Why do they come to Sri Lanka? The sea floor drops away very steeply just off the southern coast. The deep water brings up nutrients that feed lots of krill, and the whales follow the food. The area near Mirissa is one of the few places where you can see a blue whale from the shore.
Blue whales talk to each other in deep, low rumbles that travel for hundreds of kilometres through the ocean. Scientists think two whales on opposite sides of the Indian Ocean might be able to send each other 'messages'. We still don't know exactly what they are saying.