Perhaps you have once dug into the Earth o see what was down there. When you are digging you may have noticed how cool the dirt in the hole was. From this observation you may have concluded that the deeper you go, the cooler the Earth’s temperature is.
Another way to learn about Earth’s temperature is to go down into a cave. Cave visitors often comment on how cool it is down there.
Both of these observations are biased and do not give an accurate idea of the temperature below the Earth surface. The reason is that such observations are usually made in the summer. People who dig hole or go inside a cave in the winter usually observe that the temperature inside the earth is much higher than on the surface.
Geologists have observed that the temperature of the Earth about 15 meters below the surface remain the same all year long. The heat of summer and cold of winter do not affect the temperature at this depth. the temperature remains the same as the average temperature on the surface hence caves in New Mexico for example are warmer than cave in Kentucky.
Mines and wells both provide information about the temperature of the earth. the deepest mines in the world are in South Africa one such mine is over 300 meters deep. at that depth, the temperature is 51 degree C.
The deepest holes on the Earth have been made by oil well drillers. in 1972 a oil well was drilled in Oklahoma to a depth of 9160 meters. Geologist estimate that the temperature increases about 25 degree C for every kilometer of depth.