The icecap of the Mýrdalsjökull glacier covers an active volcano called Katla. The caldera of the volcano has a diameter of 10 km (6.2 mi) and the volcano erupts usually every 40–80 years. As the last eruption took place in 1918, scientists are monitoring the volcano very carefully as they believe an eruption of Katla is on the cards, particularly after the eruption of Eyjafjallajökull began in April 2010. Since 930, 16 eruptions have been documented. –Wikipedia
Update:July 9, 2011-According to the Icelandic news, a glacier flood started around midnight in Múlakvísl Glacier River that runs from Mýrdalsjökull glacier. Last time this type of event did happen was in the year 1955, when a minor eruption is believed to have taken place under the Mýrdalsjökull glacier. I do not know if that is the case now; as this might just be hydrothermal water being released due to increased activity in hydrothermal areas that are in Mýrdalsjökull glacier. Following the flood, there is a strong smell of sulphur and there might be dangerous gases in it. At current time, the flood appears to be growing and according to the news (written at 01:54 UTC) the bridge was still safe. But there was just 1 meter from the river and up to the floor of the bridge. –Jonfr.