Snowball Earth or Icehouse Earth refers to times when the Earth's surface was nearly or entirely frozen. The occurrence of Snowball (or Slushball) Earths is still controversial. However, it is probable that widespread glaciation occurred in periods of the Proterozoic.

What is debatable is how widespread those glaciations were. Proponents claim the theory explains sedimentary deposits of glacial origin at tropical latitudes and other strange features of the geological record.

Opponents do not draw the same inferences from the geological evidence, and doubt the geophysical feasibility of an entirely ice or slush-covered ocean.^[1]

• Huronian glaciation 2,400 – 2,100 million years ago (mya)

The Snowball Earth hypothesis explains glacial deposits in the Huronian supergroup of Canada. The palaeomagnetic evidence, which suggests ice sheets at low latitudes, is contested.^[2][3] The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were deposited at tropical latitudes.^[4]

The rise of free oxygen occurred during this part of the Paleoproterozoic. Perhaps purple bacteria removed methane from the atmosphere through oxidation. As the Sun was notably weaker at the time, methane, a powerful greenhouse gas, had kept surface temperatures above freezing. In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred.^[3] This happened because the methane gas and carbon dioxide in the air first mixed with water vapour. Then it precipitated as acid rain, and made carbon chemicals when the acid rain reached water. Afterwards, the carbon chemicals form limestone deep underwater. Cyanobacteria that was active at the time also took part in the freezing process by converting carbon dioxide to oxygen. The ice itself made its expansion unstoppable. Glaciers reflect 85% of the light coming toward it, and also refects the light's energy. The lack of energy and heat from the sun makes the glacier stuck in a cycle of growing. ^[5]

The period of the extraordinary ice age, Snowball Earth finally ended when a series of volcanic eruptions occured. The volcanoes made holes through the ice sheets. The greater effect these erruptions made are the mass releasing of carbon dioxide into the air. The planet reheated, and Earth was saved from being a ball of ice. ^[6]

Cyanobacteria and other forms of life might have died out in Snowball Earth. But the bacteria hid in warmer areas, inside caves. The tempertures were still a bit cold and sunlight was very scarce. But the DNA of the bacteria had a special mechanism. The DNA changed its structure when the bacteria needed to adapt. When the ice on snowball Earth melted, the percentage of oxygen in the atmosphere increased from 1% to 21%. This is so, because in the time of Snowball Earth, cyanobacteria adapted to the darkness of caves, where sunlight is scarce. When Snowball Earth ended and the cyanobacteria spread out of their caves, they were surprisingly better at photosynthesis (the converting of carbon dioxide and light (photons) to sugar and oxygen) than before. The great amount of light after the long period of darkness was enough for the cyanobacteria to make huge progress.^[7] With more oxygen, living things were able to develop more complex anatomies, and evolved to animals now, and us humans also.

• Kaigas ice age 825 – 730 mya
• Sturtian ice age 720 – 635 mya
• Marinoan ice age 650 – 635 mya

There were three or four significant ice ages during the late Neoproterozoic.^[8] Of these, the Marinoan was the most significant, and the Sturtian glaciations were also truly widespread. These were both in the Cryogenian period, before the Ediacaran.^[9] The million year long Gaskiers glaciation did not lead to global glaciation,^[10] although it was probably as intense as the late Ordovician glaciation. The status of the Kaigas glaciation or "cooling event" is unclear. Some do not recognise it as glacial, and others believe it may indeed be a third ice age. It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Evidence does suggest that the Earth underwent a number of glaciations during the Neoproterozoic.^[11]