NOVELTY - The underground biogas plant in which a petroleum residue (3) in a reactor chamber (2) is transferred with a biological preparation via pipings (4, 7) and pumping arrangement (5). The petroleum residue is converted to methane gas by aerobic reaction without formation of hydrogen sulfide, and the methane gas is passed into a processing arrangement (8) through another piping. The reactor chamber is designed in underground cavities, capillaries, porous storage or caverns and connected with above ground arrangement, pumping arrangement and/or processing arrangement via the pipings.    USE - Used for the preparation of biogas.    ADVANTAGE - Simple underground biogas plant, which utilizes complete petroleum residues for obtaining high profitable methane. The conversion of the petroleum residues into the methane gas takes place without formation of hydrogen sulfide affecting human and environment.    DETAILED DESCRIPTION - The underground biogas plant in which a petroleum residue (3) in a reactor chamber (2) is transferred with a biological preparation via pipings (4, 7) and pumping arrangement (5). The petroleum residue is converted to methane gas by aerobic reaction without formation of hydrogen sulfide, and the methane is passed into a processing arrangement (8) through another piping. The reactor chamber is designed in underground cavities, capillaries, porous storage or caverns and connected with above ground arrangement, pumping arrangement and/or processing arrangement via the pipings. The pipings are led in a common earth bore of the reactor chamber to the surface of the earth. The processing arrangement is implemented by a block heat power plant for heat and electricity generation. The conversion of the petroleum residues into the methane gas, comprises supplying biological preparation for fermentation, transferring the preparation to exhausted petroleum mine and supplying the preparation to the petroleum residues present in the caverns of the mine. The biological preparation is a mixture of water and sodium alginate.    DESCRIPTION OF DRAWING(S) - The figure shows underground biogas plant:    Soil; (1)    Reactor chamber; (2)    Petroleum residue; (3)    Pipings; (4, 7)    Pumping arrangement; (5)    Processing arrangement. (8)