All around the Martian south pole, there are huge deposits of frozen carbon dioxide, better known as dry ice. In this image by NASA's Mars Reconaissance Orbiter, we can see huge dry ice pits... and the mysterious walls that encircle them.

At first glance, it may look like these pits are lined with gold, and that's actually a possibility. At the very least, we still have no idea what the composition actually is of the dust that covers the walls of these pits. We have a much better understanding of the dry ice within the pits. Unlike the more familiar type of ice, dry ice doesn't melt into a liquid. Instead, it transitions straight back into gaseous carbon dioxide, a process known as sublimation.

The carbon dioxide starts to sublimate towards the end of each Martian summer, when temperatures get hot enough to unlock the frozen gas. However, this is only a temporary reprieve, and temperatures quickly dip back down again, causing the carbon dioxide to refreeze. The pits themselves are pretty huge - to give some sense of scale, that central circle is about two hundred feet across.