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Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.
As a result, alternating bands of lighter and darker ice can be seen in an ice core.
Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.
The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core.
It can make some snow sublimate, leaving the top inch or so less dense.
Early cores were often collected with hand augers and they are still used for short holes.
Hand augers can be rotated by a T handle or a brace handle, and some can be attached to handheld electric drills to power the rotation.
With the aid of a tripod for lowering and raising the auger, cores up to 50 m deep can be retrieved, but the practical limit is about 30 m for engine-powered augers, and less for hand augers.
The bubbles disappear and the ice becomes more transparent.
The weight above makes deeper layers of ice thin and flow outwards.