Katla hasn’t erupted since 1918, but apparently it has a much larger magma chamber. The last time it did pop its top it lasted for around a month—that’s not good—and even melted ice sheets, causing flooding across some of the country’s farmlands.
The problem seems to be that scientists are noting some earthquake activity in and around the area, and that’s what's making them a little nervous—and concerned. Especially since the tremors have been increasing in strength, and that’s never a good sign. Authorities have already been talking with residents of some smaller communities near Katla in a sort of “just in case” situation.
To summarize, this thing is a big, bad volcano, and unfortunately it’s definitely overdue for an eruption sooner than later. If—or when—it erupts it’s going to be bad for those living in and around the area first and foremost, but after that it’s likely that we’ll see a repeat of last year’s big ash problems.