Source: http://arstechnica.com/science/2015/02/s...

Almost three years ago, the Red Planet put on a bit of a show for anyone with a telescope big enough, and an eye trained enough, to spot it. As the Terra Cimmeria region of Mars’ Southern Hemisphererotated into view, a faint bulge rose above the smooth curve of the planet’s surface. It looked like a cloud, but it was too tall and too weird.

Starting on March 12, 2012, amateur astronomers reported seeing the odd lump on the Martian horizon. Reports continued to pour in over the next 11 days as the lump became even more obvious. It petered out some time before April 1, but a second occurrence was observed between April 6 and April 16. Each time, its form varied from day to day, and it was seen as dawn swept across the region—but not at dusk.

Although the Mars Reconnaissance Orbiter imaged the area daily, it did so in the afternoon, and nothing showed up.