In 1999, the cicadas of Brood V emerged in parts of Maryland, Ohio, Pennsylvania, Virginia, West Virginia, and New York, and then disappeared almost as quickly as they came, leaving only their eggs and molted exoskeletons behind. Once the eggs hatched, the new generation of cicada nymphs crawled underground, where they’ve spent the past 17 years biding their time and living off of fluid from tree roots.

Sometime this week, when the time is right and the soil is warm, they’ll emerge again to molt, enter their adult stage, mate, make a lot of noise, and lay their own eggs. (Brood V was due to emerge sooner, but scientists say the cool spring has delayed them.)

Not all cicadas play this long game of hide-and-seek. Most North American species are “annual cicadas” whose broods emerge every summer and have unsynchronized, two-to-five-year life cycles. Only a handful of species are “periodical cicadas” that have longer, synchronized life cycles, bursting forth together in huge broods every 13 or 17 years.

Seventeen years is a long time to hang out underground. Why do these cicadas spend so much time out of sight and out of mind? And why do they come out all at once?

Periodical cicadas have had scientists scratching their heads for centuries. As one team of researchers explained, “We do not know the answers to these questions but experimental evidence and mathematical models have enabled us to develop some ideas.”

One explanation for the cicadas’ long development times is that the 13- and 17-year cycles keep broods in the same region from emerging at the same time or too quickly after each other, which minimizes competition for resources and prevents interbreeding.

Another idea is that the cycles protect the cicadas from predators and parasites with shorter life cycles. Biologist Stephen Jay Gould explained it like this:

“They are large enough to exceed the life cycle of any predator, but they are also prime numbers (divisible by no integer smaller than themselves). Many potential predators have 2–5 year life cycles. Such cycles are not set by the availability of periodical cicadas (for they peak too often in years of nonemergence), but cicadas might eagerly be harvested when the cycles coincide. Consider a predator with a cycle of five years; if cicadas emerged every 15 years, each bloom would be hit by the predator. By cycling at a large prime number, cicadas minimize the number of coincidences (every 5X17, or 85 years, in this case).”

Staying out of sync with predators’ life cycles keeps the cicadas from becoming a reliable source of food. (It also prevents the predators from adapting or devising better ways of feeding on the cicadas.) This hypothesis is difficult to test because cicada emergences are so far apart, but mathematical models developed by researchers support the idea.

The long cycles might also be the result of North America’s prehistoric climate. Periodical cicadas evolved during a time when glaciers advanced and retreated over what is now the eastern U.S. Temperatures would have been unpredictably warm or cool and often too low for the insects to fly or mate. When researchers calculated the chances of survival for cicadas with different life cycles in this kind of climate, they found that the longer the insects stayed underground, the lower the chance they’d emerge during a too-cool summer. Over time, the scientists suggest, insects with shorter cycles died out, while those who by chance took longer to develop survived and reproduced.

There are other explanations for the broods’ impressive synchronization and overwhelming numbers. Some scientists suggest that high population densities are necessary to produce the deafening choruses that males use to attract mates. Another idea is that there’s safety in numbers. Cicadas don’t have much in the way of defenses, but when billions emerge at the same time, there are simply too many for predators to eat them all. Even after birds and other animals have had their fill, there are plenty of cicadas left to mate, lay eggs, and start the 17-year process yet again.