The largest animal in the world is the blue whale (Balaenoptera musculus). Blue whales are marine mammals that belong to the baleen whale suborder and are found in all of the world's oceans. They are known for their enormous size, with adult blue whales reaching lengths of up to 100 feet (30 meters) and weighing as much as 200 tons (180 metric tons).

Blue whales have a long, streamlined body with a bluish-gray coloration that is mottled with lighter spots. They have a small dorsal fin and two flippers that are up to one-third of their body length. The mouth of a blue whale is very large, measuring up to 4 feet (1.2 meters) wide, and contains between 300 and 400 baleen plates that they use to filter their food from the water.

Blue whales feed primarily on krill, a tiny shrimp-like creature that is abundant in the cold waters of the polar regions. They can consume up to 4 tons (3.6 metric tons) of krill per day during the summer feeding season.

Despite their massive size, blue whales are graceful swimmers and can reach speeds of up to 30 miles per hour (48 kilometers per hour) when swimming at full speed. They are also known for their vocalizations, which can be heard over long distances and are used for communication and navigation.

Blue whales were once hunted to near extinction, with an estimated 360,000 individuals killed during the 20th century. Today, their populations are slowly recovering thanks to conservation efforts and hunting bans that have been put in place by many countries.

In conclusion, the blue whale is the largest animal in the world, with a size and weight that is difficult to comprehend. Their massive size and graceful movements make them a true wonder of the natural world.