Scientists from the U.S. Geological Survey, from Hungary, from the European Space Agency have all proposed explanations for the black spots on Mars.

The leading one is so weird!

Imagine this: Every spring, the sun beats down on a southern region of Mars, morning light melts the surface, warms up the ground below, and a thin, underground layer of frozen CO2 turns suddenly into a roaring gas, expands, and carrying rock and ice, rushes up through breaks in the rock, exploding into the Martian air.

(via Are Those Spidery Black Things On Mars Dangerous? (Maybe) : Krulwich Wonders… : NPR)