cold period unscathed. Proponents counter that it may have been possible for life to survive in these ways: In reservoirs of anaerobic and low-oxygen life powered by chemicals in deep oceanic hydrothermal vents surviving in Earth's deep oceans and crust; but photosynthesis would not have been possible there. Under the ice layer, in chemolithotrophic (mineral-metabolizing) ecosystems theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of volcanism or geothermal activity. In pockets of liquid water within and under the ice caps, similar to Lake Vostok in Antarctica. In theory, this system may resemble microbial communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 100 m thick, and at the temperatures predicted by models equatorial sublimation would prevent equatorial ice thickness from exceeding 10 m. As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period. In small regions of open sea water: polynya. These natural ice holes can occur due to the action of winds, currents or a local heat source (e.g. geothermal), even if the surrounding sea is completely frozen over. They could preserve enclaves of photosynthesizers (not multicellular plants, which did not yet exist) with access to light and to generate trace amounts of oxygen, enough to sustain some oxygen-dependent organisms. It is not necessary that a hole form in the ice, merely that some parts of the ice become thin enough to admit light. These small regions may have occurred in deep ocean, far from the supercontinent Rodinia or its remnants as it broke apart and drifted on the tectonic plates. In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice. Such environments may have covered approximately 12 per cent of the global surface area. In small oases of liquid water, as would be found near geothermal hotspots resembling Iceland today. In nunatak areas in the tropics, where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt