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During the earlier times of the Earth plants and animals [had lived|lived|were living] on the surface until they eventually died and decomposed. They [would accumulate|were accumulating|accumulating] on the bottom of the global seabed. We know that those plants and creatures eventually [transformed|transforming|been transformed] under heat and pressure into fossil fuels. They [rot|had rotten|were rotting] and [were moldering|had moldered|was moldering] while temperature and pressure [was increasing|increased|were increasing]. After a very long period of time, mineral cover [had formed|is formed|forms] on top of this mass and it began [squeeze|being squeezing|squeezing] oil and gas more and more. Rocks [have trapped|were trapping|had trapped] gas and oil in their pores and this is how oil and gas reservoirs [took|have taken|had been taking] shape under the earth. This treasure [lie|had been lying|would lie] deep beneath the surface for quite a long time. Since prehistoric times, these underground reservoirs with fuel [had held|holded|has held] tremendous amounts of fossil fuels until the time came to use them. Only two natural elements such as hydrogen and carbon [mix|mixing|are mixed] together [create|creating|to create] fossil fuels till the end of that process. The fuel [had been storing|stored|had stored] more and more energy until people discovered it. Its original strength [would come|is coming|came] from the sun and [has changed|changed|was changinging] to natural burning substance. Millions of years [had passed|were passing|had been passed] and people [were not noticing|had not noticed|were not noticed] the sources of energy inside the earth till the discovery of fire. Fossil fuel suddenly [had become|would become|became] necessary.