What will happen when methane is at a temperature of 1500 K? On the first glance the answer seems to be obvious - methane will decompose into hydrogen and one of the forms of carbon. Yes. However is does not do so at very high pressure, when novel reaction pathways become possible. The latest experimental results and theoretical calculations show that methane and heavier hydrocarbons are, remarkably enough, stable under extreme pressures and temperatures. Even more, experiments confirm the possibility of abiogenic synthesis of natural gas at 5.0 GPa and 1500 K. The review summarizes published results of theoretical and experimental investigations of possible pathways under the conditions of pressure and temperature that prevail in the Earth's upper mantle for the formation of (1) particular species of hydrocarbon molecules, and of (2) complex hydrocarbon systems. The results raise fundamental questions on the genesis of hydrocarbons.