Easter Island

Background
Easter Island, also known as Rapa Nui and Isla de Pascua (Spanish for Island in the Middle-of-fucking-nowhere) is famous for being the most isolated human settlement on earth, and for being the isolated civilizations to completely kill itself in the shortest amount of time (shortly followed by the inhabitants of the seldom-discussed Bioshphere 3).

Anthropologists estimate that the first people arrived on Easter Island in around 980 AD, carried by wooden canoes, sea turtles, rafts made of animal carcasses, and, when they could afford them, motorboats. The following history of the island is what made the Europeans so terrified that they named it “Easter Island”, in hopes that a huge rabbit would appear and crush the island and the terrible memories associated with it under a giant egg.

The Ancestor Cult
The people of Rapa Nui made their lives simply, by fishing, growing yams, and hunting the native land-shark. General boredom and lack of motivation became a frequent problem, and to combat this, people developed several religions. One of them that rose to power was called the ancestor cult, and grew popular by solving the common problem of how to talk to the dead (in case you needed to consult someone about growing yams or hunting land-sharks, and no one around was helpful). The presented answer was, to modern civilization, laughably obvious: carving humongous heads out of stone and propping them up on hillsides in random directions to weird out future generations.

As the Easter Islanders had no cranes or trees taller then the stone heads from which to hang pulleys, they had to resort to filling the space under the heads with babies to get the heads standing upright. Frequently the stone heads were so heavy that it would take several generations-worth of babies before they could sufficiently communicate with the dead long enough to solve their yam-growing problem.

The Birdman Cult
Many, however, felt that the giant stone heads were simply not badass enough for Easter Island, and created a new method of talking to the dead: jumping off a 200-foot-tall cliff into the sea, swimming across miles of shark-infested water to a tiny islet, stealing the egg of the sooty nesting tern (Birdus insignificantus), swimming all the way back, climbing said cliff again without breaking the egg, and crowning whoever came back alive as king.

This was, of course, hugely popular, especially since going to the islet meant one would hear the chanting of the nameless bird-god who lived there, which would give them control over all flying things (the bird-humans returning from the islet were, still at risk from land-sharks). The Birdman Cult shortly displaced the Ancestor Cult, and several birdmen were crowned kings before the civilization self-destructed.

The End
The end of the powerful Easter Island society came around slowly. Stone head popularity began to rise again as prices fell, and the island was strained for resources, even with the new shacks for full-time production of new babies to raise the heads up. Between this and the continually waning land-shark populations, Rapa Nui was already strapped for resources.

The final hit came when the sooty nesting terns realized that if they just applied themselves more and went back to school, they could probably get a job and move out of their mother's basements and maybe get a studio apartment in the city, then they could finally find that loving, cute, single sooty nesting tern they had been dreaming about, who was funny but not a bitch and was into experimentation, and so left for the South American mainland in a fit of self-realization.

The Easter Islanders, overcome with boredom again, were reduced to fighting. Despite (and perhaps because of) the newest technological advance of giant catapults to fire the stone heads at each other, the entire population of Rapa Nui was at one point reduced to 36: about the size of your high school English class, plus the group of nerds you used to eat lunch with and that girl from your cousin's party.
The result, as you can imagine, was so terribly awkward that it was practically a relief when European explorers showed up with their empty slave-collars and their pneumonia, small-pox, and ebola-aids.

Today, minor volunteer-run programs are in place on Rapa Nui to bring back the valiant Easter-Island culture: including using imported babies to prop catapulted stone heads up, and restoring the land-shark to its native frolicking grounds with careful breeding and release.