One of the easier mysteries, if more chilling, is the mystery of the missing population between the visits of Admiral Jacob Roggeween and Captain Cook in 1774. The accepted explanation is that the islanders had outgrown their resources: agriculture couldn't feed the growing population. They cut down the trees, and without the means to build canoes and leave the island, they eventually resorted to war and cannibalism. The moais were pulled down as first one faction then the other destroyed their statues. Many theorists see what happened on Easter Island, label it as Rapa Nui Syndrome, and see it as a warning to the rest of Earth's population.
The abiding mystery are the Moai statues of Rapa Nui. What are they? Why are they? Who are they? One prevailing theory is that each of moai is a representation of the god and ancestor, and as in other Polynesian religions, gave power, or mana, to the people who erected and maintained the statue. If, as theorized, each of the family or clans on the island, had their own moais, building a platform called an ahu to serve as a family burial vault, then it is easy to understand why warring clans would want to destroy the source of each other's power.
This theory doesn't explain the placement of the moais, nor why some look so different than the ones with prevalent long ears, thin lips and unsmiling expressions. Traditionally, the warring factions have been identified as Short Ears and Long Ears, which may explain the greater number of long-eared statues. Then there is the mystery of the missing eyes. Were the eye sockets carved out and left empty until the moai was erected and the mana supposed to begin working, or were the eyes, made of coral and scoria inserted only on ceremonial occasions?
Thor Heyerdahl expostulated that the settlers of Rapa Nui came by balsa raft from South America. His book Kon-Tiki created a wave of interest and permission to excavate and examine some of the moais. Theorists since then have either supported his work, as in Linguistic Evidence of Early Peruvian-Rapanui Contacts or refuted entirely the idea that humans had anything to do with the moais. In The Space Gods Revealed, Erik Von Daniken put forth the theory that bored space aliens created the statues. Neither theory is substantiated by the archaeological evidence although perhaps the NOVA team who attempted to erect a statue using only the tools a native inhabitant would have, might have welcomed some outside help. Read their story in Secrets of Easter Island. All the moais now standing were re-erected over the last decades.
As the moais were toppled or abandoned, and no new ones created, the culture shifted to what is now called the cult of the BirdMan. This was still in existence, and documented in the 1860's and more than 150 carvings or petroglyphs exist in the rocks around the ruins of the village of Orongo, near the caldera of Rano Kau. The carvings depict a man's body with a bird's head, sometimes holding an egg in one hand, and the theory exists that this cult demonstrates the desire to escape the island. The basic ceremony of this cult was the task to find the first egg laid each spring on an offshore island by Manu Tara, a sacred bird. Each clan chief sent one candidate, or hopu, to swim to Moto Nui, the largest island below Orongo, there to wait for the eggs to be laid. When the hopu found an egg, he strapped it to his forehead and then made the dangerous swim back, climbed the cliffs and presented the unbroken egg to his chief. This chief would become BirdMan for the coming year, with powers and privileges. Some of the petroglyphs have fertility symbols mixed in. At the other end of the island is an area thought to be a solar observatory, or an astronomy tower.
The Rapa Nui had a form of writing called rongorongo which no one could decipher. The meaning and source of these enigmatic characters has been open to interpretation for years, since a tablet was sent to Tepano Jaussen, Bishop of Tahiti, as a token of respect, by newly converted islanders.