Three years ago on March 20, in southeastern Pennsylvania, an amateur astronomer named Wayne Jaeschke turned his telescope to the skies and captured something strange. There was some sort of blob or serpentine vapor-trail emanating from the surface of Mars. Not knowing what else to call it, he named the phenomenon in a subsequent blog post: “a martian stumper.”

“At first I thought: Maybe there’s a pixel error in the live feed on my computer, or some dust has collected on a [telescope] sensor,” he told Popular Mechanics. He soon told other amateur astronomers to check his work. Were they seeing what he was seeing? Eighteen did, confirming this wasn’t some trick. There really was an ethereal plume swirling off Mars’s southern hemisphere. It wasn’t long before the experts picked up the scent and tried to figure out what the oddity could be. Was it a cloud? Or perhaps aurora like the Earth’s own?

Now, those researchers have published their findings in the journal Nature. But the findings have only added another layer of mystery. Even now, following months of study, no one is any closer to identifying what precipitated the plume. Scientists can note few facts beyond its apparent diameter: more than 600 miles. “It raises more questions than answers,” Antonio Garcia Munoz of the European Space Agency and a co-author of the study told the BBC.

The edge of the red planet is “a unique window into the complex atmospheric phenomena occurring there,” the study said. “… Importantly, explanations defy our current understanding of Mars’s upper atmosphere.” The plume is simply too high, co-author Agustin Sanchez-Lavega said in a statement, calling it “extremely unexpected.”

What little scientists know: The plume materialized out of nowhere in March, “became more prominent over the following days,” and didn’t remain static over the following 11 days when amateur astronomers had clear sight of it. “Remarkably, the aspect of the features changed rapidly,” the study said, “their shapes going from double blob protrusions to pillars or finger-plume-like” shapes.

An early hypothesis had it that this was a cloud. It hung around 150 miles from the ground, after all. But this was substantially higher than any other cloud had ever reached in Mars’s atmosphere.

“We know there are clouds on Mars,” Garcia Munoz told the BBC. But 60 miles is usually the outer limit, he said. Sanchez-Lavega agreed, saying that it would require “exceptional deviations from standard atmospheric circulation models” for something like that to be possible.

So could it be light? According to Popular Mechanics, probably not. Although the shaft did occur in an area of Mars with a strong magnetic field, it wasn’t nearly as bright as it should have been if it had been an aurora.

So for now, scientists are mulling explanations — but more frequently discarding them. “I’ve heard about four or five different possible explanations,” Bruce Jakosky, a NASA planetary scientist, told Popular Mechanics. “And honestly? I don’t like any of them. … The takeaway is that we’ve seen something totally unexpected that doesn’t have a simple explanation.”