Mr Illes said a layer of soil 2cm deep (0.8 inches) would need to be removed from the whole of the contaminated region.

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That seed is germinating deep beneath an obscuring layer of thick, black soil.

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Soil and plants also keep the sun off the roof layer itself, making it last longer.

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C. and A.D. 500 by brushing away the dark top layer of barren desert to reveal the light, sandy soil underneath, wrote Clive Ruggles, an archaeologist from the University of Leicester in the United Kingdom, in an email.

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The mountain is a giant, putrid layer-cake, with dozens of strata of rubbish separated by soil and plastic liners designed to contain the brew of noxious chemicals that would otherwise leach into groundwater.

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Filter material: layer of fine gravel used in slope stabilization structures that allows water to escape and retains soil particles.

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