Termite Shelter Tubing
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When a source of food (wood) is not actually touching the soil, termites must build shelter tubes between the soil and the wood to protect them from sun and air currents, and to maintain the high relative humidity that they need to survive.
Ironically, these tiny insects who can literally eat you out of house and home are actually very delicate. Without the protection afforded by shelter tubing, they wouldn't be able to survive the trip between the soil and a piece of wood only a few inches away.
Termite tubing is remarkably effective. Some years ago, after chlordane was banned, various highly repellent termiticides were introduced, none of which worked very well. One reason was that termites began building shelter tubing through the treated soil to avoid contact with the termiticide. (That doesn't happen with Termidor, the liquid termiticide that we use. Termidor is completely non-repellent to termites.)
Large colonies can produce very intricate networks of shelter tubing, which often are the first visible signs of an infestation. Unfortunately, many people mistake visible tubing for mud or mortar (which is one more good reason to have your home inspected every so often by a qualified termite control expert).