Coal and petroleum have been produced from large amounts of biomass
entrapped inside the earth under high temperature, pressure and
anaerobic conditions. Such a situation develops only rarely like big
upheavals on earth. At present no more coal or petroleum is being
formed. All that is available has been formed millions of years ago.
Being rich source of energy, coal and petroleum are being consumed in
ever increasing amount in industry, transport, kitchens, etc. If the
trend continues, soon they will be exhausted. Therefore, they must be
conserved by developing more efficient machines, hybrid engines and
using hydrogen as a fuel.