The sea is salty. In fact a gallon of sea water contains, on average, about a quarter of a pound of ordinary table-salt. But where, one might wonder, did it come from?
The reason provided in Norse mythology, and believed for centuries, was that somewhere at the bottom of the ocean, grinding continuously, was a huge, gigantic, magic salt mill. But in the 17th century, our own Robert Boyle provided a more convincing explanation: he suggested rivers as the source of sea salt.
The "fresh" water of the world's rivers and streams contains small amounts of salts such as sodium and chloride, leached from the rocks and soils in its path. These flow with the water to the sea, and add their tiny modicum to the quantities there. Boyle's theory was that when water evaporates from the ocean's surface the salts are left behind.
Indeed the same body of water can be imagined as continually circulating - evaporating, condensing into clouds, and falling as rain into rivers and lakes which will eventually contribute to their ocean another pinch of salt.
With salt continually being added, the seas have gradually become the way they are.
This explanation satisfied nearly everyone for centuries. But then scientists began to spot some inconsistencies. For one thing, it is relatively easy to estimate the average salt content of the world's rivers, and then calculate how long it would take for the sea to reach its present level of salinity.
It turns out to be a surprisingly short amount of time - only 100 million years - much less than the known age of the Earth.
This simple theory also implies that the sea must be getting saltier as time goes by, but ancient sediments show that the ocean had about the same salinity 200 million years ago as it has now.
And finally, if material was being continually leached from land like this, in a couple of hundred million years or so, there would be no land left to leach.
The complex answer has two main strands. Firstly, the ocean, rather than being a simple receptacle for the materials washed into it by all its rivers, is a great chemical factory which reprocesses its influent in different ways.
And secondly, tectonic plate theory seems to provide a mechanism where material deposited on the ocean floor is subsumed into the bowels of the Earth, and thrown up again over millions and millions of years to provide a fresh supply of land.
Complex, indeed, are these two strands, but as Shelley says: "Yet both so passing strange and wonderful!"