Here's why:

* Early Earth: Earth was initially very hot and lacked a stable atmosphere or oceans.

* Volcanoes: As the planet cooled, volcanic eruptions released large amounts of gases, including water vapor, into the atmosphere.

* Condensation and Precipitation: This water vapor eventually condensed into clouds, leading to precipitation and the formation of the first oceans.

While some water may have arrived on Earth through icy comets and asteroids, the vast majority of our planet's water originated from volcanic outgassing.