NASA engineers are using a duplicate version of the Mars rovers here on Earth to try and test strategies Opportunity could use to dig its way out of a sand dune. The rover bogged down during a drive on April 26, and controllers have asked it to just stay put while they work on the best way to escape. The team is experimenting with a mixture that they think mimics the composition of the sand in the dune, and hope to put what they've learned to the test next week.