In 1999, NASA's Mars Polar Lander, sent to the unexplored south pole of Mars, was lost just before it was supposed to land. It is unknown what happened, but it's assumed the debris (or the simply uncommunicative probe) is on the surface of Mars.
The Mars Reconnaissance Orbiter has been taking lots of incredible high-res images of the planet, including the likely areas where MPL hit. However, that's a lot of data to sift through! So the HiRISE folks have put the data together, and they're asking you, me droogs, to help them look. Maybe you'll be the first person to see the doomed spacecraft since it was lost!