Opportunity's sand box....

Here's JPL's note about working on the way out of the stuck position
Opportunity is in:
http://www.jpl.nasa.gov/news/news.cfm?release=2005-072

quote
"We choose to proceed cautiously, so we don't expect to begin actually
driving out of the dune before next week, possibly later," said Jim
Erickson, rover project manager at JPL

[...]
Last week, engineers arranged a simulated dune using sand that was
already at JPL's rover testing facility and put a test rover into a
comparably dug-in position. The test rover had no difficulty driving
away, even when sunk in belly-deep. However, that sand offered better
traction than the finer, looser material that appears to make up the
surface at Opportunity's current position. "We needed to do tests using
material more like what Opportunity is in, something that has a
fluffier texture and cakes onto the wheels," said JPL rover engineer
Rick Welch, who is leading the tests.

Experimenting with different mixtures, engineers and scientists came up
with a recipe that includes play sand for children's sandboxes,
diatomaceous earth for swimming pool filters and mortar clay powder.
Then they went to several home supply and hardware stores to find
enough bags and boxes of the ingredients to make more than 2 tons of
the simulated Mars sand for more realistic mobility tests, said JPL
rover mobility engineer Jeff Biesiadecki.

[...]
Experiments indicate that in this more powdery material, the test rover
positioned comparably to Opportunity can drive out after some initial
wheel-spinning. More testing, analysis, planning and review will
precede any actual commands for Opportunity to begin driving away from
the dune
/quote