That's a great question. Until we invent a way to go back in time -- and find someone who is brave enough to go back 4.6 billion years -- scientists can only guess. We've made our guess based on what we know about the Earth and other planets, and based on how we think our solar system formed. Scientists always base their ideas on physical evidence: things we can measure test and observe. God lies outside this sphere. So science is just one way of looking at things. I'm going to give you the scientific explanation.

Scientists know the Earth is made up of layers: a brittle, thin crust on the outside; a thick, soft layer in the middle (the mantle) and an incredibly hot, metallic center (the core). We think the temperature of the inner core (the center of the Earth) is about the same temperature as the surface of the sun. Wow!!! Where does that heat come from? One source is natural nuclear energy. The amazing pressure at the center of the Earth from all that overlying rock causes nuclear fusion reactions, which release heat. The rocks in the mantle and crust trap the heat and keep it from escaping. A second source of that incredible amount of heat at the Earth's center is very old: heat left over from when the Earth first formed, about 4.6 billion years ago.

We think the planets formed from collisions of space dust and gas that were rotating around the sun. (Where does space dust come from? Exploding stars! Because we are made from the same material the Earth is made of, we are all star dust.) For rocky planets, like Earth, the dust stuck together and became small meteors, the small meteors stuck together and became large meteors, the large meteors crashed together and became small planets, and the small planets crashed together and became the planets we see today. The fact that the plants weigh so much (gravitational attraction) shapes them into spheres instead of jumbled chunks of rock. All that crashing released enormous amounts of heat, and at some point, the Earth was so hot from these collisions that ALL the rock melted, and the Earth was a hot ball of melted rock orbiting the sun. The heavy material (metals) sank to the center and the lighter material (silicate minerals, like in quartz and other gem stones) floated to the top. Eventually, the Earth cooled off, and a hard, cold, brittle crust formed.

At this point, there was no oxygen in the atmosphere, and no water on the surface. All the water within the surface rocks had evaporated. Slowly, volcanoes formed as they do today and when they erupted, they burped up water vapor from the melted rock in the mantle (silicate minerals contain water). This water vapor hit the cold atmosphere and condensed and came down as rain. After a long time, the oceans filled up and became salty. Some of this water also came from collisions with comets, which are giant ice balls with rocks in the center. When the comets hit the Earth, the ice melted on impact and became water vapor, which condensed in the atmosphere and came down as rain. When did the oxygen in our atmosphere show up? When life evolved. (Bacteria capable of photosynthesis evolved about a billion years ago and produced some oxygen. About 700 to 500 million years ago, seaweed, algae and eventually land plants evolved and made a whole lot of oxygen.)

So the short answer is: so hot everything melted.

The early Earth was incredibly hot - it was composed entirely of magma (liquid rock) for hundreds of millions of years.

The Earth coalesced from asteroids and comets that hit each-other and agglomerated. This produced a huge amount of energy - the planet was almost certainly entirely molten when we would first describe it as a planet.