There's no shovel or drill powerful enough to allow scientists to plumb the depths of Earth to reach its mantle – the 1,800-mile-deep layer sandwiched between a relatively thin crust and the planet's dense core.

This is unfortunate, because Earth's geologic history is recorded in solid rock that appears motionless to our transient eyes but flows like honey if viewed over millions of years. Scientists call this movement the mantle wind.

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Geologists know that the uppermost 250 miles of Earth's mantle are not uniform, but rather show distinct groupings of rock, called domains, identified by their distinct chemical signatures. 

But how many domains exist (right now, scientists know of three) and where exactly their borders lie is still a mystery. As they're studied, these domains can provide valuable clues about Earth's ancient climate, how carbon is recycled, how the continents have reshuffled across the globe and much more.

New research, published in Nature Communications Earth and Environment, reveals that the one domain –  the Zealandia-Antarctic domain, known to be located roughly beneath New Zealand, Australia and Antarctica – actually stretches further north of Australia and west of New Zealand and is 23 million square miles large, which is twice the size of Africa and possibly three times larger than previously recognized. 

Jonny Wu, a University of Arizona associate professor in the Department of Geosciences whose lab coordinated the research, co-authored the paper with the University of Nevada Reno's Jeremy Tsung-Jui Wu (no relation) and lead author Shengping Qian from China's Tongji University.  

"This mantle domain appears to have its distinctive geochemical signature because it was mostly isolated under oceans and did not have much interaction with continents traveling across it in the past 400 million years," Wu said. Pristine material from the mantle can be tainted as continents move across the surface of the Earth.

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Wu and his colleagues were tipped off that there might be something unexpected beneath the crust when their collaborators in China harvested rocks from the Philippine Sea Plate, a tectonic plate that lies beneath the western Pacific Ocean, using a remote-operated submersible. Wu and his team also dug into a database of all the rocks from the mantle that had erupted on the Philippine Sea Plate – Earth's fastest moving tectonic plate for the last 65 million years – and found that some rocks weren't where they ought to be based on their current understanding of mantle domains. 

Once they created a map of the rocks at their ancient eruption locations, a picture emerged of the three mantle domains.   

"What's cool about this paper is that we found a plate that moved across a large part of the mantle, crossing all three mantle domains that we know about," Wu said. "We don't get many opportunities to probe the mantle's secrets. It was just fortuitous." 

They found that the domain that lurked beneath has probably been around since the Paleozoic Era, a time before the dinosaurs roamed, over 250 million years ago. This is important because scientists can use more stable mantle domains like this one as a reference against which to compare more dynamic movement of the mantle.  

"When we try to rewind the clock to travel back into geologic time, we need references," Wu said. "But the issue is that most things have been moved around so much in the mantle that that's hard to do." 

If geologists can show that these domains are long lasting, they can provide a clearer map of the mantle wind and how its movements have spanned the globe over its long history.