Some dead organisms are converted into fossil fuels, such as coal, gas, and oil, over geologic time. Typically this carbon is trapped and remains outside of the carbon cycle. Small amounts of carbon from these sources may slowly reenter the cycle as water interacts with them.However, modern human behavior has made a practice of burning large amounts of fossil fuels, releasing carbon back into the atmosphere as carbon dioxide. Factories, cars, and fuels to heat human homes all produce carbon dioxide. Burning wood has a twofold impact because it removes an organism capable of taking up carbon dioxide and releases more carbon dioxide into the atmosphere when the wood is burned. Carbon cycles in aquatic environments include the same biological processes. Large amounts of carbon are stored in limestone from ancient ocean creatures buried millions of years ago. As these rocks erode, carbon is released into aquatic environments. In addition, carbon dioxide can also diffuse into water directly.