At any given time there are roughly 20 active volcanoes around the world, and there are 16 volcanoes called "Decade Volcanoes;" that are currently noted to have a history of large eruptions and a proximity to populated areas. Imagine the cost if one of those erupts.
The 2010 eruptions of Eyjafjallajokull caused drastic interference to air travel across most of Europe. The second eruption of the year on April 14th produced massive ash plume that made it impossible for pilots to travel to their destinations. Here is a recap of the details and its effects on the travel industry.