Earth formed around 4.54 billion years ago[3][4][5] by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and ocean, but the atmosphere contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. One very large collision is thought to have been responsible for forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.