To keep the Olympic torch burning at the summit of Everest, Chinese engineers have had to overcome two main challenges: the weather and the atmosphere. Average temperatures range from minus 36C (-33F) in January to minus 15C in July, and it is also buffeted by jet-stream winds that can reach hurricane force.

Atmospheric pressure is only a third of that at sea level, which means that only a third of the normal amount of oxygen is available to burn. Together, these issues make it extremely difficult to burn any kind of fuel at such a high altitude.

The thin atmosphere means that an independent source of oxygen would be required, much as liquid oxygen has to be carried by rockets fired into space so that combustion can take place.

The low temperatures would make pressurised gas used as fuel less volatile and harder to burn, and high winds could blow out an unprotected flame.

Although details of the Chinese design are not precise it is known that engineers used liquid propane, normally used as camping gas, as fuel. This would have to be supplemented with compressed oxygen to allow it to burn. Professor Hans Michels, of Imperial College, London, said: “You would have a differential valve that feeds gas from both canisters into the burner at the required rate.”

The propane canister would also have had to be heated and a pressure-retaining valve would have protected the flame from the wind.