Actual anthills, of a specific, but widespread, species of ant. Known as harvester ants, they are red, and have a nasty sting. In the northern cold country, they build conical hills, that are hollow. They move their larvae in and out of the mound to control their temperature. They also use the mound to keep the seeds they live on, dry. Don't want them to germinate. To protect the mound, they range out a couple hundred yards, bringing back heavy minerals to place on the mound to protect it from the weather. By checking the mound, you are seeing the minerals that are in the surrounding area, without wandering over the entire countryside. The mounds may be as much as 50 years old, and should not be destroyed. Bad Karma!. The native Americans said anything removed from a mound should be replaced with something else. I've found many garnets, ilmenite, and chrome diopside on mounds in Wyoming. Have yet to find gold or platinum, however, but the ants are apparently genetically programmed to bring back the heaviest items they can locate. Obviously, they can't carry anything very big, so you need a really sensitive detector, like the Falcon, to locate a tiny piece of ant-sized gold. Only problem with the Falcon is the tiny search head.
Jim