Scientists have struggled to explain how a 'magic' island appeared in the middle of a lake on Saturn's moon Titan - and then disappeared.
Titan is one of the largest Moons in our solar system, and the biggest orbiting the ringed planet.
It is of particular interest to astronomers because it has both a thick atmosphere, a rocky surface and surface liquid lakes. While the lakes are made of methane and ethane, rather than water, they are able to exist as liquid at temperatures lower than 180C and make it unique among known worlds other than our own.
Various proposals for exploring Titan have been made in recent months and weeks, including a mothership and drone team, and even a submarine.
But the depth of our ignorance surrounding Titan was made even more clear on a recent fly-past of the space probe Cassini-Huygens.
On the flyby of the Moon in 10 July 2013, Cassini spotted a new island in the middle of Ligeia Mare, the second-largest lake on the world.
But when the craft made its next pass around the planet it had vanished.
Scientists have proposed various explanations in a new paper published in Nature Geoscience, but say its appearance is still largely unexplained.
"'Magic island' is a colloquial term that we use within the team to refer to this. But we don't actually think it's an island," co-author Jason Hofgartner told the BBC.
Among the potential solutions are that the 'island' was a giant wave, a bubble of gas, a floating 'solid' island or a 'suspended solid'.
Scientists think the island -- whatever it is -- was caused by the seasons which extrapolate on the Moon over the course of 30 years.
During the 'summer' on Titan (which is about to start) winds get stronger because more of the Sun's energy is directed at the northern hemisphere. That also leads to larger waves, which might be why the island appeared. It might also lead to the melting of icebergs made of methane and ethane.
Head over to the BBC for a full explanation of the possible theories, or read the original report over at Nature Geoscience (subscription required).