About three percent of the weight of sea water is accounted for by salt. The salt content of the ocean is being continually added to by runoff from the land, but it is not increasing. Yet salt cannot leave the sea by evaporation because the water vapor leaves it behind. So for many years it was a mystery as to why the sea should have so little salt in it, given the rate of runoff, and particularly if the world was supposed to be hundreds of millions of years old. The riddle was not solved until the 1970s, when the deep ocean openings were discovered.
What does <was supposed to> mean in the context?