Narwhals can dive to 1,800 metres

They may bear more than a passing resemblance to the mythical unicorn, but narwhals are very much real and are a member of the porpoise family that lives exclusively in the Arctic and northern Atlantic oceans.
With a fairly compact body, a super-thick layer of blubber and the ability to dive as deep as 1,800 metres (5,900 feet) for seabed-dwelling halibut, they are perfectly adapted to surviving in the frozen north.
Their most distinguishing feature – the spiral tusk which can reach up to three metres (9.9 feet) on males – is not a horn but a tooth which grows through the upper lip. The tusk’s purpose is not known for certain, but the general consensus is that it’s used primarily for courtship and mating rituals. Males are sometimes seen crossing tusks in a practice called ‘tusking’, but this is believed to be play-fighting – a way of establishing a social hierarchy within the group (similar to a stag’s antlers), or even a method of cleaning the teeth, rather than a serious attempt to maim/kill.
Like most cetaceans, narwhals live in pods, though their size can vary from just a few individuals up to several hundred or even several thousand, especially during migration season.