Primordial rain

In the beginning of the times the Land, the sun and the moons had just formed. The Sun became very hot and started to melt the moons. The Silver Moon melted and what rained down became the seas. The Red Moon melted and what rained down became the blood of the living beings. Over time the water will sometimes evaporate and go back to the Silver Moon, but the blood will sink to the ground, which is why the Red Moon is smaller. This is also why the sacrificed blood must be burned to return to the Moon.