The Earth is moving more than 3,000 miles under our feet, according to new research.

Its inner core goes back and forth over a mile - every six years. The cycle explains variations in the length of days - which have oscillated for decades.

Professor John Vidale, of the University of Southern California, said: "From our findings, we can see the Earth's surface shifts compared to its inner core, as people have asserted for 20 years."

The huge iron ball is the size of Pluto - and as hot as our sun. The US team used seismic data from 1969 to 1974 to create a computer model of it.

Simulations contradicted previous theories suggesting consistent rotation faster than at the planet's surface. The latest observations show the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74

Utilizing data from the LASA (Large Aperture Seismic Array), a US Air Force facility in Montana, Vidale found the inner core rotated slower than previously predicted - approximately 0.1 degrees per year

Lab staff developed a novel beam-forming technique to analyze waves generated from Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago Novaya Zemlya.

Measuring the compressional waves resulting from the nuclear explosions, they discovered the inner core had reversed direction, sub-rotating at least a tenth of a degree per year.

By using seismological data from atomic tests in previous studies, the researchers have been able to pinpoint the exact location and time of the very simple seismic event.

Vidale said "The inner core is not fixed - it is moving under our feet, and it seems to going back and forth a couple of kilometers (1.25 miles) every six years.

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