Caption and image courtesy of the Snowball Earth Web site: Sea-glacier flowage means that crack systems will perpetually exist wherever the moving ice impinges on the sea floor or landfast ice. Cracks propagate through thick ice due to water pressure and freezing produces juvenile (<5 years old) sea ice hosting brine channels populated by a variety of prokaryotic and eukaryotic, phototrophic and heterotrophic organisms. Because oxygen cannot diffuse from isolated brine channels, the organisms must be tolerant of hyperoxia, a potentially useful pre-adaptation if atmospheric oxygen levels rose following the younger Cryogenian snowball earth.