Easter Island's famed statues have long captivated our imagination - if not just our practical minds.
How did the monolithic giants get there? How were they built?
Now a new study has worked out how their Polynesian architects were able to mount 13-tonne red hats on their heads.
"Lots of people have come up with ideas - but we are the first to come up with an idea that uses archaeological evidence," Pennsylvania State University's Sean Hixon said.
The island, sitting in the middle of the Southern Pacific Ocean, is thought to have been first inhabited in the 13th century by Polynesian travellers.
Its famous and mysterious statues, carved from volcanic tuff, came from one quarry on the island, while the hats, made of red scoria, came from a different quarry on the other side of the island.
Previous research has determined that the statues, which can be up to 10m tall and weigh 81 tonnes, were moved into place along well-prepared roads using a walking and rocking motion, similar to the way a fridge might be moved.
Not all statues made it to their final places, and the fallen or broken ones showed that, to move them, the statues were carved so they leaned forward and were later levelled off for final placement.
The hats, each weighing 13 tonnes, might have been rolled across the island, but once they arrived at their intended statues, they still needed to be lifted on to the statues' heads.
The islanders probably carved the hats cylindrically and rolled them to the statues before further carving the hats to attain the final shapes, which vary from cylindrical to conical and which usually have a smaller cylindrical projection on the top.
Chips of red scoria are found in the platform of some of the statue hat combinations.
"We were interested in figuring out the method of hat transport and placement of the hats that best agrees with the archaeological record," Hixon said.
The researchers took multiple photographs of many hats to see what attributes were the same throughout.
Using photogrammetry and 3D imaging, they created images of the hats with all their details.
"We assumed they were all transported and placed in the same way," Hixon said.
"So we looked for features that were the same on all the hats and all the statues."
The only features they found the same were indentations at the bases of the hats, and these indentations fit the tops of the statues' heads.
If the hats had been slid in place on top of the statues, then the soft stone ridges on the margin of the indentations would have been destroyed, so the islanders must have used some other method.
Previous researchers suggested that the statues and the hats were united before they were lifted in place, but the remnants of broken or abandoned statues suggested this wasn't the approach used.
More likely, the hats were raised to the top of standing statues using large ramps.
But even so, once the hat was at the top of the ramp, it could not simply be pushed into place because of the ridges on the margin of the hat base indentation.
Rather, the researchers believe that the hats were tipped up on to the statues, where they could be rotated using levers.
"This is the first time anyone has systematically explored the evidence for how the giant hats were placed on the top of the heads of the massive statues of Easter Island," said Binghamton University's Professor Carl Lipo.
"Our work combines cutting-edge 3D modelling with artifact analysis and models drawn from physics to arrive at the best answer."
What Easter Island can tell us about climate change
Meanwhile, another strange study partly drawing on Easter Island asks whether sustainability as we know it is even possible for civilisation.
Astronomers have inventoried a sizable share of the universe's stars, galaxies, comets, and black holes.
But are planets with sustainable civilisations also something the universe contains?
Or does every civilisation that may have arisen in the cosmos last only a few centuries before it falls to the climate change it triggers?
A group of researchers have just tackled the question - at least from an astrobiological perspective.
"Astrobiology is the study of life and its possibilities in a planetary context," said Professor Adam Frank, of the University of Rochester.
"That includes 'exo-civilisations' or what we usually call aliens."
Frank and his colleagues point out that discussions about climate change rarely take place in this broader context - one that considers the probability that this is not the first time in cosmic history that a planet and its biosphere have evolved into something like what we've created on Earth.
"If we're not the universe's first civilisation, that means there are likely to be rules for how the fate of a young civilisation like our own progresses."
As a civilisation's population grows, it uses more and more of its planet's resources.
By consuming the planet's resources, the civilisation changes the planet's conditions.
In short, civilisations and planets don't evolve separately from one another - they evolved interdependently, and the fate of our own civilisation depended on how we use Earth's resources.
To illustrate how civilisation-planet systems co-evolve, Frank and his team developed a mathematical model to show ways in which a technologically advanced population and its planet might develop together.
By thinking of civilisations and planets - even alien ones - as a whole, researchers can better predict what might be required for the human project of civilisation to survive.
"The point is to recognise that driving climate change may be something generic," Frank said.
"The laws of physics demand that any young population, building an energy-intensive civilisation like ours, is going to have feedback on its planet.
"Seeing climate change in this cosmic context may give us better insight into what's happening to us now and how to deal with it."
Using their model, the researchers found four potential scenarios that might occur in a civilisation-planet system.
They included mass "die-off" survived by only a fraction of the world's population; a shift to a sustainable future that avoided the most catastrophic effects; a population collapse that came without any change to the world's resources; or a population collapse that came in spite of a late shift to sustainability.
"The last scenario is the most frightening," Frank said.
"Even if you did the right thing, if you waited too long, you could still have your population collapse."
The researchers created their models based in part on case studies of extinct civilisations, such as the inhabitants of Easter Island.
People began colonising the island between 400AD and 700AD and grew to a peak population of 10,000 sometime between 1200AD and 1500AD.
By the 18th century, however, the inhabitants had depleted their resources and the population dropped drastically to about 2000 people.
The Easter Island population die-off related to a concept called carrying capacity, or the maximum number of species an environment can support.
The Earth's response to civilisation building was what climate change is really all about, Frank said.
"If you go through really strong climate change, then your carrying capacity may drop, because, for example, large-scale agriculture might be strongly disrupted."
In the meantime, he issued a sober warning.
"If you change the Earth's climate enough, you might not be able to change it back," he said.
"Even if you backed off and started to use solar or other less impactful resources, it could be too late, because the planet has already been changing.
"These models show we can't just think about a population evolving on its own.
"We have to think about our planets and civilisations co-evolving."