According to the passage, the limited supply of oxygen underground ----.

Contrary to popular belief, underground fires are a
surprisingly frequent phenomenon, the fuel being coal
and the fire travelling along the seams, or the thin layers
of rock or mineral. Such fires travel slowly due to the
limited supply of oxygen, but can burn for a very long
time: the underground fire at Burning Mountain Nature
Reserve in Australia is thought to have been continuing
for the past 5,500 years. The number of such
subterranean fires worldwide is countless. According to
one study, subterranean fires in China alone are
consuming some 200 million tonnes of coal a year and
pumping into the air as many pollutants as all the cars in
the United States. Along with numerous human-related
factors, such fires are also contributing substantially to
global warming.