There are different kinds of Cicadas, and each species has its own life cycle. The Cicadas that we see every year actually live for 2 to 5 years underground, before emerging as adults. They are not all on the same cycle, so we see them every year. Other Cicadas have much longer life cycles, 13 or 17 years, and they tend to all be in sync, so they all appear at the same time, although some areas have more than one group, with different timing.