A 15-kilometre wide patch of seagrass could be the oldest living thing on earth.

Researchers including Carlos Duarte, of the University of Western Australia analysed 40 meadows across 3500 kilometres of the Mediterranean Sea.

The seagrass, giant Posidonia oceanica, reproduces asexually, generating clones of itself.

The giant organism, which is more than 6000 tonnes in mass, is thought to be more than 100,000 years old.

This seagrass is declining at an estimated rate of 5 per cent annually.

"The concern is that while Posidonia oceanica meadows have thrived for millennia, their current decline suggests they may no longer be able to adapt to the unprecedented rate of global climate change," Professor Duarte said.

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