Earth's oceans were once green until oxygen-producing bacteria turned the water blue and the land red.

During the Archean Eon, Earth's atmosphere lacked oxygen, allowing iron to dissolve easily into the oceans and giving the water a distinct green hue. This changed approximately 2.4 billion years ago during the Great Oxidation Event. Cyanobacteria, the first organisms to perform oxygenic photosynthesis, began releasing massive amounts of oxygen into the environment as a byproduct.This newly introduced oxygen reacted with the dissolved ferrous iron in the sea. This chemical reaction created insoluble ferric iron, commonly known as rust, which precipitated out of the water and settled onto the ocean floor. These massive deposits eventually formed Banded Iron Formations (BIFs), which are the primary source of iron ore mined by humans today.As the iron was stripped from the water, the oceans transitioned from green to their modern clear blue. Simultaneously, the oxidation of minerals on the Earth's surface turned the landscape red. This biological and chemical shift was one of the most significant transformations in Earth's 4.5 billion-year history, paving the way for the evolution of complex life that depends on oxygen.