Enormous clouds that erupted 260km above the surface of Mars in 2012 are still baffling scientists.

In a new report in the journal Nature, a team of scientists led by Agustin Sánchez-Lavega have concluded that none of the usual culprits for such an atmospheric change can be deemed responsible. Not water, carbon dioxide ice, dust storms, or auroral light could have existed at that altitude, Sánchez-Lavega and his team determined.

Sánchez-Lavega declared that any “explanations defy our current understanding of Mars’ upper atmosphere” and their investigation only furthers the intrigue surrounding these mysterious clouds.