Such ice cores trap gases, bits of dust, and other chemicals that were present in the snow that fell over Greenland for thousands of years and then became compressed into ice.

A tourist trap par excellence, this drugstore in South Dakota is famous for its roadside billboards that start advertising 'free ice water' several states away but it's a surprisingly worthy stop.

That produced a greenhouse effect which, in turn, melted seabed structures known as methane clathrates, which trap that gas in ice.