The twin Mars rovers, Spirit and Opportunity, have observed more than 4,000 geographical features on the planet since they landed in January 2004. Expected to run for only three months, the rovers are still going strong, and their mission has now been extended to at least September of next year. They've been ranging for so long, in fact, that they're causing a problem for their handlers. In order to document the terrain efficiently, scientists have had to come up with unique names for all the features the rovers discover, a task that gets tougher with each passing day.