evidence: Glaciers form as layers of snow accumulate on top of each other. Each layer of snow is different in chemistry and texture, summer snow differing from winter snow. Over time, the buried snow compresses under the weight of the snow above it, forming ice. Particulates and dissolved chemicals that were captured by the falling snow become a part of the ice, as do bubbles of trapped air. Layers of ice accumulate over seasons and years, creating a record of the climate conditions at the time of formation, including snow accumulation, local temperature, the chemical composition of the atmosphere including greenhouse gas concentrations, volcanic activity, and solar activity. [...] At sites where the ice is well below freezing, such as the interiors of the polar ice sheets, mechanical drills must be used. In contrast, thermal drills are particularly effective at coring through warmer ice (e.g., ice approximately above -10°C) and are frequently used on mid- or low-latitude glaciers. On many non-polar glaciers, however, ice conditions can vary from "cold" ice to "warm" ice, requiring both mechanical and thermal drilling to retrieve the best possible core quality over the entire depth interval that is drilled.