Phase two would cover an area of over 16 sq m (40 sq km) along the eastern boundary of the phase one area.

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"Think of it as a fuzzy boundary between the mantle and the core, with patches perhaps 10 to 20 km (6 - 12 miles) across and up to a 1, 000 metres (3, 280 ft) thick, " Jeanloz said.

BBC: Earth's wobbles reveal deep secrets

The origin of this effect may lie at the boundary between the mantle, made up of viscous rock that extends 2, 900 km (1, 800 miles) below the crust, and the outer core, which is thought to be liquid iron with the consistency of water.

BBC: Earth's wobbles reveal deep secrets