The blue whale (Balaenoptera musculus) is the largest animal known to have ever lived on Earth. It is a difficult whale to study because of its low numbers and its preference for deep, offshore waters. Within Balaenoptera musculus, authorities recognize between three and five subspecies. Blue whales live in oceans throughout the world, including off the Atlantic and Pacific coasts of Canada. Although blue whale sightings are rare, experts believe that about 250 mature individuals live off each coast.