Water is indeed one of the most common and abundant substances on Earth, covering about 71% of the Earth's surface and believed to make up nearly two-thirds of the human body. However, water is not the most common substance in the entire Earth system.

The Earth's Crust:

Within the Earth's crust, which is the outermost layer, the most common elements by mass are oxygen (47%) and silicon (28%), followed by aluminum, iron, calcium, sodium, potassium, and magnesium. These elements combine to form rocks, minerals, and compounds that make up the solid part of the crust.

The Mantle and Core:

The Earth's mantle, located beneath the crust, constitutes a significant portion of the Earth's volume and contains a substantial amount of magnesium and iron. Iron is particularly prevalent, accounting for approximately 85% of the core's mass.

In terms of the most common substances in the entire Earth, including its core, mantle, and crust, iron stands out as the most abundant element by weight. Oxygen remains abundant as well, making up a substantial portion of the crust and mantle.

So, while water is vital to life on Earth and covers a large surface area, it does not hold the distinction of being the most common substance in the entire planet.