A mysterious patch of water in the Pacific Ocean hasn’t touched the surface since the fall of the Roman empire.
Experts used computer modelling of deep sea currents to reveal the reason why the vast ‘shadow zone’ has remained near stagnant for around 1,500 years.
They found that it sits in between layers of water with currents driven by heat from the Earth below and whipped up by wind above.
The unique shape of the ocean floor means that upwards currents don’t reach high enough to push the layer upwards, leaving it in a no man’s land between the two.
An international team of researchers, including the University of New South Wales (UNSW) and Stockholm University, studied the strange region, between 0.6 and 1.5 miles (one and 2.5 km) down.
It covers an area 3,700 by 1,250 miles (6,000 by 2,000 km), where the North Pacific meets the Indian Ocean.
Carbon dating has previously been used to identify its age and location, but scientists didn’t understand what caused it to form.
By including the shape of the ocean floor in theirsimulation, the team was able to measure its impact on the movement ofcurrents.
They found that water at the bottom of the ocean, heated by geothermal energy deep within the planet, was unable to rise above1.5 miles (2.5km) below the surface.
Instead of travelling upwards, currents loop back on themselves horizontally, leaving the layer directly above untouched.
Dr Casimir de Lavergne, lead author from UNSW, said: ‘Carbon-14 dating had already told us the most ancient water lied in the deep North Pacific.
‘But until now we had struggled to understand why the very oldest waters huddle around the depth of 1.2 miles (2km).