Yes, the ground only freezes near the top. Deeper down itīs hot enough to melt rock. Basically the thickness of this layer depends on your lattitude and altitude. I live at about 50° North at 1000' of altitude above sea level. Around here we consider about 35" as safe. Closer to the equator the ground may not freeze at all and further north youīll find permafrost. There the ground is frozen long and deep enough to preserve entire mammothes. At higher altutudes you may also find frozen ground to be very deep.
Short story: Get information about your area and act accordingly.
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If it isnīt broken, it doesnīt have enough features yet.