The bomb squad that attended the scene measured the crater to be 33 feet wide and 14 feet deep. Using this data, they were able to confirm that the culprit would have been a WW2 500lb high explosive bomb. Furthermore, given the history of the area, it was likely to be American, as opposed to a British weapon. The crater was therefore the result of 262lb (119kg) of TNT (high explosive).
The spontaneous explosion is not thought to have been caused by any outside stimulus. A local government spokesman said it is believed that the bomb had a chemical-based time delay fuse which finally decayed, triggering the explosion. The fuse would have been originally designed to function anywhere from a few hours up to a few days.
The United States Eighth and Ninth Air Forces attacked a nearby railway marshalling yard several times during March 1945. Even by this late stage in the conflict, bombing was still relatively inaccurate with munitions dropped miles away from their intended target. There are also a number of other reasons why this bomb was so wayward.
Is there a risk of a similar event in the UK?
During WW2, Germany dropped tens of thousands of 500lb bombs on the UK. However, the Luftwaffe’s general purpose bombs used either electrical impact or clockwork (time delay) fuses. They did not use chemical fuses. Consequently, an incident such as this would not occur in the UK. A buried German bomb would still require a shock, most likely from intrusive engineering works, to initiate the detonator.