Good question. The theory is that, when the disk of dust (water and gas) slowly came together to form Earth, our planet was extremely hot. However, instead of the water evaporating from its surface, it's believed that Earth housed this water deep below its surface in an underground reservoir near its mantle. Once the crust cooled and hardened, the water in the reservoir seeped out through volcanic vents, like Iceland's lava fields, in the form of steam. Rising, this steam later condensed and fell down as rain once Earth's atmosphere moistened. According to Hallis, geologically speaking, it would not have taken long for the Earth to transition from a dry surface to a wet one. So, if premordial lava has a lower D/H ratio than the ocean, then how did Earth's water come to have a higher ratio than the water from the lava it come from? To answer this, Hallis believes that, overtime, "heavy water-rich" comets and asteroids did collide with Earth to increase the D/H ratio of the ocean. In addition, the solar wind, a stream of particles (protons and electrons) that shoot from the Sun's atmosphere and can fling away molecules (ions) from a planet's upper atmosphere, is largely credited to "whisking away" water vapor that contains "regular" hydrogen because it's easier to "blow away."