"Borehole temperatures" refers to the measurement of temperatures deep underground by drilling a hole (borehole) into the Earth's surface. Full definition
There are now improved and expanded data since the TAR, including, for example, measurements at a larger number of sites, improved analysis of borehole temperature data and more extensive analyses of glaciers, corals and sediments. (ipcc.ch)
This study integrates the complementary information preserved in the global database of borehole temperatures [Huang et al., 2000], the 20th century meteorological record [Jones et al., 1999], and an annually resolved multi proxy model [Mann et al., 1999] for a more complete picture of the Northern Hemisphere temperature change over the past five centuries. (realclimate.org)
Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide. (usap-dc.org)