Scientists and astronomers have been left scratching their heads after a huge cloud was spotted over the southern regions of Mars.
The massive plume was spotted in 2012 and lasted 10 days before disappearing and then re-appearing a second time. It hasn't been witnessed again since.
Researchers have been studying the images since then and have finally published their findings in the science journal 'Nature'.
Whatever it is, it's enormous. Between 500-1000km in diameter the huge cloud was spotted at an altitude of between 150-200km.
The reason it's puzzling scientists is because it simply shouldn't be there. Speaking to BBC News, Dr Garcia Munoz explains why.
"We know there are clouds on Mars, but clouds, up to this point, have been observed up to an altitude of 100km, and we are reporting a plume at 200km, so it is significantly different. At 200km, we shouldn't see any clouds, the atmosphere is too thin - so the fact we see it for 20 days in total is quite surprising."
If it isn't a cloud then what else could it be? One other theory is that it could be an example of Mars' aurora, however this again would cause some significant problems with what we already know as Dr Munoz goes on to explain.
"We know in this region on Mars, there have been auroras reported before. But the intensities we are reporting are much much higher than any auroras seen before on Mars or on Earth. It would be 1,000 times stronger than the strongest aurora, and it is difficult to come to terms that Mars has such an intense aurora."
So those are our options: It's either a gargantuan cloud appearing where clouds shouldn't appear, or an aurora that's 1,000 times strong than any other seen on the planet, neither of which scientists are particularly keen on because as Munoz points out in the paper, "both explanations defy our current understanding of Mars’ upper atmosphere."
The hope is that by sharing everything they know so far, the wider scientific community can come up with either some alternatives, or a theory that can prove either of those options.
It's either that or we'll have to wait for the mysterious cloud to appear again. Or you know, we can just have a look when we get there.