NPR’s Robert Krulwich writes:
What are those things? They were first seen in 1998; they don’t look like anything we have here on Earth. To this day, no one is sure what they are, but we now know this: They come, then they go. Every Martian spring, they appear out of nowhere, showing up — 70 percent of the time — where they were the year before. They pop up suddenly, sometimes overnight. When winter comes, they vanish.
As the sun gets hotter, they get more spidery.
Krulwich explains that we still don’t know what these are, but the leading explanation is that they are geysers kicking up black dust. From the surface of Mars, they might look a little something like this: