Despite the fact that scientists have lived largely ignorant of these cones, they've been around for quite a long time. Funch dated 11 samples from all over northeastern Brazil with a relatively new technique that uses cosmic ray strikes as a natural clock. As particles from deep space slam into crystals inside the mounds, electrons get knocked loose and stuck in new locations at a predictable rate. When researchers heat or illuminate the crystals in a lab, the electrons fall back to their original locations, giving off a faint flash. Based on the number of these flares, the team determined that the mounds sampled ranged from about 700 to nearly 4,000 years of age. In future studies, Funch hopes to sample more mounds to get a more accurate range, and also different layers within a single mound, which would give an idea of how long a mound takes to form.