The ravine has 400m-high walls of sheer limestone, which block out the sun and silence the world outside.

BBC: On the trail in Sardinia

This is the full-on, block-out-the-sun, Cormac-McCarthy business.

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The result is a material that is not only waterproof but is able to block out UV radiation from the sun to such an extent that it would rate a Ultraviolet Protection Factor (UPF) of 101.15 if the chart went that high.

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