In April 2004, "Mean Girls" was playing in theaters and "Yeah!" by Usher was topping the Billboard music charts. This spring -- 17 years later -- those cicadas are part of Brood X (ominous as the "X" sounds, it stands for the Roman numeral ten) and for all that time they have been underground eating and growing. Researchers aren't sure exactly how many will surface, except that it will be in the billions: They estimate the numbers will be at least 1.5 million per acre, which could mean as many as 30 of the creatures covering your average square foot.