Those trying to communicate across the seas using a message in a bottle had better hope it doesn't get caught in a patch of stagnant water in the north Pacific that could mean it takes 2000 years to resurface.

The so-called shadow zone of very slow overturning circulation – stretching about 6000 kilometres west to east, 2000 kilometres north to south, and lying 1-2.5 kilometres below the ocean's surface – is home to the world's oldest water, researchers including several based at the University of NSW have found.

Satellite imagery of part of the north Pacific Ocean. Credit:NOAA

The rough location of the water with unusually extended periods between contact with the atmosphere has been known for some time. The new research, pushed recently in Nature, helps explain the processes involved that keep the water from circulating back to the surface for 1500 or perhaps even 2000 years.

"Our main advance is actually to understand how and why this water is so old," said Ryan Holmes, from the ARC Centre of Excellence for Climate System Science, and one of the paper's authors.