Why does the ground a few feet below the surface stay at a fairly constant temperature?

The ground a few feet below the surface maintains a relatively constant temperature because the annual fluctuations in air temperature are dampened as they penetrate the soil. While surface temperatures vary significantly with the seasons, these fluctuations diminish rapidly with depth.1

At a depth of approximately 9 to 15 meters (30 to 49 feet), the ground reaches a "level of zero amplitude," where seasonal temperature changes are barely detectable. This stability occurs because the soil acts as an insulator, and the time required for heat to conduct through the earth filters out the rapid, short-term variations seen at the surface.12

Below this depth, the influence of seasonal surface weather disappears, and the temperature begins to increase steadily due to the geothermal gradient—the natural flow of heat from Earth's interior, generated primarily by the radioactive decay of elements like potassium, thorium, and uranium within the crust and mantle.13