By Roland Pease
BBC science unit, San Francisco
Scientists say they have witnessed the possible birth of a future ocean basin growing in north-eastern Ethiopia.
The team watched an 8m rift develop in the ground in just three weeks in the Afar desert region last September.
It is one small step in a long-term split that is tearing the east of the country from the rest of Africa and should eventually create a huge sea.
The UK-Ethiopian group says it was astonished at the speed with which the 60km-long fissure system developed.
"It's the first large event we've seen like this in a rift zone since the advent of some of the space-based techniques we're now using, and which give us a resolution and a detail to see what's really going on and how the earth processes work; it's amazing," said Cindy Ebinger, from Royal Holloway University of London.
Professor Ebinger and colleagues described the event here at the American Geophysical Union Fall Meeting.
In the far-distant past, oceans such as the Atlantic have formed when supercontinents have torn apart.
Indeed, North America and Europe are still moving in opposite directions at about the pace fingernails grow.
Researchers have long recognised that the Afar region, an inhospitable depression in north-eastern Ethiopia, has been contorted by similar forces in recent geological time.
But the event in September is said to be unprecedented in scientific history.
It began with a large earthquake on the 14th of the month and continued with a swarm of moderate tremors.
Start and stop
"About a week into the sequence, there was a volcanic eruption," explained Dr Ebinger.
The huge numbers of tremors have displaced thousands of people
"A lot of ash was thrown up in the air, and a lot of cracks appeared in the ground; some of which were more than a metre wide.
"Using satellite techniques we can see ground deformation, and about a month after the sequence, we could see a 60km long section had opened up, and it opened up about 8m in its central part.
"It appears that we've seen the birth of an ocean basin."
The movements of September are only a small part of what would be needed to create a whole ocean - the complete process takes millions of years - and in other regions of the planet, ocean development has been started only to stall at a later time.
But the Afar event has given geologists a unique opportunity to study the rupture process at close quarters.