Suggestion: 60% of intelligent Runeterrians live above ground

Namis and Fizz' people live underwater, and so do up to 35% of humanoid creatures.

For clarification, Riot never said this....but they never said it wasn't the case

Because on the bottom of the Oceans of Runeterra it is quiet. And wet. We look up and see Fishman McJohnson wiggling his amphibian extremities, propelling him to his destination: some spooky old underwater grave. No fish in this Water would​ be stupid enough to get lost in this terrifying tomb, after all, his fish-elders explicitly told him about this place being the single most otherworldly and nightmare inducing palace of death.

But McJohnson doesn't give a fish. Slimy slithering past mossing debris at the entrance and he finds himself in a Labyrinth. No way of determining how big since the walls strech up to a whopping 29 fm (fishmeters) to the ceiling "I'll hold on the the right wall I guess" he bubbles. Labyrinths are easy. Fishman swimms through 9 submarine daycycles across the gigantic construction, like an unrelenting and majestic worm until he reached the middle of the palast.

The moment he felt the presence of that he already knew that he was marching towards his certain death. What was this creature, whose mere eye contact strikes fear and horror in mortal fishmen across the ocean. Two steps left. Behind this wall is a chained up incarnation of evil eager to meet its visitor.

One step. Gills are freezing up.

Last step . . . Mermaid Riven

Fishman McJohnson died instantly from intoxication on that night