A NASA Mars rover, the Spirit, is stuck.

The rover drove into soft ground a couple of weeks ago, and when it tried to get out, its wheels slipped and it moved only a couple of inches.

The Spirit has been stuck before, and mission managers have been assiduously driving it around potential hazards. But they did not see the soft spot, which was hidden under a veneer of normal-looking soil.

“It is quite serious,” said John Callas, the project manager for the Spirit and the Opportunity, its twin rover on the other side of Mars.

Steven W. Squyres, the principal investigator of the rover mission, said the material looked like ferric sulfate salt, which had previously been encountered. “From a rover mobility standpoint, it is insidious stuff,” he said. “This stuff has very little cohesion. What this means is that it’s very tough for the rover wheels to get a grip on it.”