http://www.jpl.nasa.gov/releases/2004/142.cfm
NASA has decided the potential science value gained by sending Opportunity into a martian impact crater likely outweighs the risk of the intrepid explorer not being able to get back out.
Opportunity has been examining the rim of the stadium-sized "Endurance Crater" since late May. The rover team used observations of the depression to evaluate potential science benefits of entering the crater and the traversability of its inner slopes.