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The Laws of The Ocean
The principal difference between this world and the real one is the existence of an extra fundamental force that acts between asymmetric molecules. It can attract or repel, depending on their relative orientations. Water is the lightest and most common such molecule, but the effect is neutralized at large scales by the molecules' random orientation. Most organisms with a circulatory system have developed the ability to rotate the water in their bodies, giving them a variety of sensory input. A very few are able to manipulate water outside their bodies, to cause changes in their external environments.
Cosmology
It's mostly like the real world--big bang, matter condensation, stars, galaxies, the works. Other than the little matter of an additional fundamental force, what sets my world apart is a major extinction event that destroyed all civilization within the ocean.
Principal Geography & Features
Oceans cover 80% of the world's surface. The relevant civilization is established in a volcanically active archipelago. They're aware of one major continent, but haven't explored much farther--although there are tantalizing hints of a much greater world than they realize.
Initial Active Setting
The story takes place mostly in a cluster of islands in a great ocean. There is contact with a small coastal region, beyond which lies a vast, unknown continent.