For thousands of years humans have cultivated useful plants and animals in large numbers and thereby enjoyed the benefits of agriculture. However, virtually all of this has taken place on the Earth's land surface. In the western world, the idea of aquaculture (marine agriculture) has only come about in the last century. Aquaculture is the controlled rearing, at high density, of marine animals (and occasionally plants) of commercial value. Following considerable investment, Australia now possesses an important new area of primary production, worth more than $105 million a year.