While piloting a commercial transatlantic flight last year, Captain Klaus Sievers and his crew got a whiff of an unusual odour. In a confined space 10 km up in the air, there was only one thing it could be.

The foul smell with traces of sulphur in the cockpit came from none other than the Grímsvötn volcano that was spewing gas and ash from southeast Iceland.

Sulphur dioxide often indicates volcanic ash, and the presence of ash in the atmosphere can endanger jet engines.