These birds, which often look like large ducks when sitting on the water, are also known as common loons. They have a haunting call which can sound like ghostly laughing, which could have given rise to the phrase, "crazy as a loon".

They are true water birds and find it difficult to walk on land because their legs are so close to their tail, which helps them to swim. Instead of walking, they slide on their bellies and push themselves along, and only head to the shore for nesting. They are aggressive birds, and during breeding will attack and sometimes kill ducks that are too close to their nests. They breed in America, Iceland and Greenland along marshy edges of lakes.

Great northern divers are powerful swimmers that dive to catch fish. They are capable of very deep dives, which almost rival that of some penguins. Some have been snared in nets at depths of over 45m.

According to Norse legend, if a flock of divers call as they fly overhead, they are following souls to heaven.