To understand where all the salt comes from, first you need to know that the Murray-Darling Basin was submerged under a shallow sea; so on a map it looked like a group of islands rather than its present shape. At that time the whole land mass started to rise out of the sea (about 100 million years ago). Salt water flowed away from the emerging land back into the ocean, and a river was born which would eventually become the present-day Murray (see the beginnings).