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. Emerging evidence suggests that Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis. Palaeoproterozoic The snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested, and stratigraphic evidence clearly shows only three distinct depositions of glacial material (the Ramsay, Bruce, and Gowganda Formations) separated by significant periods without. 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 possibly deposited at tropical latitudes. It has been proposed that rise of free oxygen that occurred during the Great Oxygenation Event removed methane in the atmosphere through oxidation. As the Sun was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing. In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred. Karoo Ice Age Before the theory of continental drift, glacial deposits in Carboniferous strata in tropical continental areas such as India and South America led to speculation that the Karoo Ice Age glaciation reached into the tropics. However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the supercontinent Gondwana. See also Europa Gaia hypothesis Global catastrophic risks Great Unconformity Greenhouse and icehouse Earth Ice age Interglacial and stadial periods Lake Nyos Milankovitch cycles References Further reading (Geol. Soc. America). External links The Snowball Earth 1999 overview by Paul F. Hoffman and Daniel P. Schrag, 8 August 1999 Snowball Earth web site Exhaustive on-line resource for snowball Earth by pro-snowball scientists Hoffman and Schrag. New Evidence Puts 'Snowball Earth' Theory Out In The Cold sciencedaily.com. 2007. Analyses in Oman produce evidence of hot-cold cycles in the Cryogenian period, roughly 850–544 million years ago. The UK-Swiss team claims that this evidence undermines hypotheses of an ice age so severe that Earth's oceans completely froze over. Channel 4 (UK) documentary, Catastrophe: Snowball Earth episode 2 of 5, first screened Dec 2008, documentary narrated by Tony Robinson, advocates snowball Earth and contains interviews with proponents. First breath: Earth's billion-year struggle for