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The geothermal gradient is the rate of increase in temperature per unit depth in the Earth. It varies with location and is typically measured by determining the bottom open-hole temperature after the drilling of a borehole. To achieve accuracy the drilling fluid needs time to reach the ambient temperature. This is not always achievable for practical reasons. In stable tectonic areas in the tropics a temperature-depth plot will converge to the annual average surface temperature. However in areas where deep permafrost developed during the Pleistocene a low temperature anomaly can be observed that persists down to several hundred metres.[1] The Suwałki cold anomaly in Poland has led to the recognition that similar thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes throughout Poland, as well as in Alaska, northern Canada, and Siberia.