Why is Australia referred to as Down Under?

[Omi]

Australia is known as 'the land Down Under' for its position in the southern hemisphere. The discovery of Australia began when European explorers searched for a land under the continent of Asia. Before Australia was discovered, it was known as Terra Australis Incognita the unknown southern land. Despite the term's wide usage, it is rarely used by Australians themselves, many of whom regard it with some derision.