TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The reaction is carried out at - 20-75 degrees C, preferably at atmospheric temperature, and at 1x 104 Pa, preferably at atmospheric pressure. Water vapor is formed during the methanation reaction of carbon monoxide (CO) and/or CO2, where the water vapor is partly adsorbed and/or condensed. The hydrogen and/or hydrogen species are directed towards the solid methanation catalyst. The feed gas is pretreated before contacting the solid methanation catalyst to reduce the water content to less than 5 (preferably less than 1) vol.%. The feed gas is pretreated before contacting the solid methanation catalyst to reduce the oxygen content or the oxygen species content to less than 10 (preferably less than 0.1) vol.%. The methanation of CO2 is operated according to a two-stage reaction pattern, where in a first stage, a feed gas comprising greater than or equal to 5 (preferably greater than or equal to 20) vol.% CO2 is contacting the solid methanation catalyst at less than 150 degrees C and at 1x 104-100x 105 Pa, where the solid methanation catalyst has a surface area of greater than or equal to 75 m2/g, and in a second stage, the formed CO species and oxygen species present on the solid methanation catalyst are reacted with hydrogen or hydrogen species at less than 150 degrees C and 1x 104-100 x 105 (preferably 1x 105) Pa. In the first stage CO2 is at least partly dissociated into CO species and oxygen species to the solid methanation catalyst, and the CO species and oxygen species being adsorbed or attached to the methanation catalyst. The reaction is carried out in a large subsurface cavity provided with a methanation catalyst, injection pipe(s) and/or tube(s) extending within the cavity.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The feed gas oxygen with a CO2/oxygen molar ratio of 98:2-94:6 (preferably 95:5), and/or CO with a CO2/CO molar ratio of 1:1-85:15 (preferably 70:30-85:15). The feed gas comprises (vol.%) nitrogen (less than 10, preferably less than 2), and an inert gas (preferably nitrogen) (greater than 80). The solid methanation catalyst is selected to catalyze the dissociation of CO2 at less than 150 degrees C and 1x 104-100x 105 Pa, and to catalyze the reaction of CO species and oxygen species to the catalyst with hydrogen or hydrogen species at less than 150 degrees C and 1x 104-100x 105 Pa. The methanation catalyst comprises at an aluminum-containing support, preferably a gamma -alumina-containing support, provided with copper (Cu), nickel (Ni), molybdenum (Mo), rhodium (Rh), palladium (Pd), platinum (Pt), or zinc (Zn). It comprises gamma -alumina-containing support provided with Rh, and with Cu, Ni, Mo, Rh, Pd, Pt, Zn, zirconium (Zr), cerium (Ce), vanadium (V), niobium (Nb), chromium (Cr), tungsten (W), manganese (Mn), iron (Fe), cobalt (Co), tin (Sn), antimony (Sb), tellurium (Te), silver (Ag), and/or gold (Au). The catalyst is an alumina, silica, and/or activated carbon containing support comprising Cu, Ni, Mo, Rh, Pd, Pt, Zn, Zr, Ce, V, Nb, Cr, W, Mn, Fe, Co, Sn, Sb, Te, Ag, and/or Au. The support is modified with Ti, Zr, and/or Ce, or with an inorganic oxide comprising alumina, silica, titanium, zirconia, barium, cerium, silica-alumina, silica-titania, silica-zirconia-titania, zirconia-ceria, and/or zirconia-barium oxide. EXAMPLE - About 3.8 g gamma -alumina were dipped in 400 mL deionized water. Ammonium chlororhodite trihydrate was added to the solution in an amount for achieving a theoretical 1 wt.% metallic rhodium on the surface of the support gamma -alumina. The solution was stirred at 20 degrees C for 3 hours, dried at 110 degrees C for 12 hours, and burned at 700 degrees C for 4 hours. The fraction of catalysts with a particle size of 200-315 mu m had a Brunauer Emmett Teller surface area of 59 m2/g, metallic rhodium content of 1 wt.%, and average diameter of the pores of 180 Angstrom , and a total porosity of 0.28 cm3/g.