possibly relating to your comment, these passages were edited for space constraints:

Today is also ‘sol 85’ for the ‘Opportunity’ rover on the Meridiani Plains, halfway around the planet. Unlike the rock-strewn rough terrain of Gusev, this region consists mostly of rolling sand dunes. But the rover has discovered other surface features, including mysterious and potentially dangerous ‘fissures’ in the ground.

These linear depressions are such unexpected features that a variety of words were used at first to describe them: depending on the document, they were called fissures, or troughs, or even “gulleys”. Squyres particularly disliked the last word since it implied formation by water erosion.

One recent rover mission status report suggested these structures (it called them ‘sand sinks’) could be signs of danger. “The science team believes that some of the depressions we're seeing on the plains may be caused by sand 'drainage' into subsurface cavities,” the report stated. “If this is the case, these areas may represent some threat if the vehicle, under its own weight, were to sink or push through the surface when traversing over what would otherwise not be mobility hazards.”

Operators convened a “working group” to look at this issue, but eventually decided there was no danger. “They don’t look very scary to me,” Squyres added, explaining that what he called ‘fractures’ were probably just cracks in the underlying bedrock into which surface sand had settled, perhaps billions of years earlier.

Orbiting spacecraft have imaged surface subsidence from collapsed lava channels (see http://www.spaceref.com/news/viewsr.html?pid=12580) but Squyres said he saw no evidence that any volcanic activity was involved in the formation of these features.