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Professor Iain Stewart visits the Mortarach glacier in the Swiss Alps. Here, geologist Louis Agassiz gathered evidence in the 1830s to prove that glaciers moved downhill. Prof Stewart re-creates Agassiz's experiment and then shows modern evidence of movement using web-cam images. Stewart is lowered into a crevasse to show the scale of a glacier, then goes into an ice cave to examine boulders embedded in the glacial ice.