Since the early 1920s, most petroleum geologists have favored a biogenic theory for the formation of oil. According to this theory, organic matter became buried in sediments, and subsequent conditions of temperature and pressure over time transformed it into oil. Since 1979 an opposing abiogenic theory about the origin of oil has been promulgated. According to this theory, what is now oil began as hydrocarbon compounds within the earth's mantle during the formation of the earth. Oil was created when gasses rich in methane, the lightest of the hydrocarbons, rose from the mantle through fractures and faults in the crust, carrying a significant amount of heavier hydrocarbons with them. As the gases encountered intermittent drops in pressure, the heavier hydrocarbons condensed, forming oil, and were deposited in reservoirs throughout the crust. Rock regions deformed by motions of the crustal plates provided the conduits and fractures necessary for the gases to rise through the crust. Opponents of the abiogenic theory charge that hydrocarbons could not exist in the mantle, because high temperatures would destroy or break them down. Advocates of the theory, however, point out that other types of carbon exist in the mantle: unoxidized carbon must exist there, because diamonds are formed within the mantle before being brought to the surface by eruptive processes. Proponents of the abiogenic theory also point to recent experimental work that suggests that the higher pressures within the mantle tend to offset the higher temperatures, allowing hydrocarbons, like unoxidized carbon, to continue to exist in the mantle. If the abiogenic theory is correct, vast undiscovered reservoirs of oil and gas – undiscovered because the biogenic model precludes their existence – may in actuality exist. One company owned by the Swedish government has found the abiogenic theory so persuasive that it has started exploratory drilling for gas or oil in a granite formation called the Siljan Ring – not the best place to look for gas or oil if one believes they are derived from organic compounds, because granite forms from magma and contains no organic sediments. The ring was formed about 360 million years ago when a large meteorite hit the 600-million-year-old granite that forms the base of the continental crust. The impact fractured the granite, and the Swedes believe that if oil comes from the mantle, it could have risen with methane gas through this now permeable rock. Fueling their optimism further is the fact that prior to the start of drilling, methane gas had been detected rising through the granite.