What are the conditions that limit the spread of life in the geological shells of the Earth?

There is a wide variety of living organisms in the underground habitat. These are plants, fungi, various animals and microorganisms. But if we investigate the geological layers, then the deeper we go, the less living organisms we will find there. All due to some factors. The deeper, the less oxygen there, but there are more different gases. Also, there is less moisture and the deeper, the hotter, since the core temperature is very high. Therefore, only a few species of bacteria can live deep underground.