Termites

They're everywhere around the Copper Belt (north-central Zambia and southern Congo). Some mounds are nearly as tall as fairly large trees. And in places, there's a mound every 50 meters or so. Here's an average mound from the road between Lubumbashi and the border.
Here are smaller mounds -- equally common in many areas. They can survive fire, apparently.
Two cross-section photos. Gross.