Scientists are reporting what they say is the longest sediment avalanche yet measured in action. It occurred underwater off West Africa, in a deep canyon leading away from the mouth of the Congo River. Something in excess of a cubic kilometre of sand and mud descended into the deep. This colossal flow kept moving for two whole days and ran out for more than 1,100km across the floor of the Atlantic Ocean. The event would have gone unrecorded were it not for the fact that the slide broke two submarine telecommunications cables, slowing the internet and other data traffic between...