The oceans were formed many millions of years ago. Water vapour thrown into the atmosphere by volcanoes condensed, resulting in rainfall. Hollow areas of the crust filled with water to form the oceans.
In the beginning, there was a huge cloud of dust and rocks in the Universe. Gravity caused the cloud to shrink and gradually the Sun and the planets formed. The original dust and rocks included minerals that had water in them. The Earth, as it formed, became very hot. It melted and there were lots of volcanoes. The water in the rocks formed steam and came out of the volcanoes as giant clouds that cooled and rained. That is where some of the water came from.
Also, the original cloud of dust and rocks included a lot of rocks that were made of ice, like giant snowballs. There are still lots of them circling the Sun. We call them comets. There were billions of them when the planets were forming and many crashed to the ground and melted. That is where the rest of the water came from. Water is made of hydrogen and oxygen atoms. (Atoms are the tiny little building blocks of the Universe – even you and I am made of atoms).
The hydrogen atoms in water formed in the Big Bang that started the Universe. The oxygen atoms were made later in stars. They got together and water was formed.
Picture Credit : Google
