Today's Image of Mars shows part of El Dorado, located in Gusev Crater, the landing site for Mars Exploration Rover Spirit back in 2004.
El Dorado is a field of dark, rippled sand that is visible from space as a dark spot on the side of Columbia Hills. The composition of El Dorado results in the low albedo feature seen from orbit. It was determined that El Dorado is a large deposit of dark windblown, "well rounded, well sorted, olivine rich" sand (Lunar Planetary Institure). While the sands of El Dorado are different than those in other parts of Gusev Crater, they are not as different as the image taken from orbit suggested.
The disparity between the image taken from orbit and the ground image is a result of people interpreting the raw images sent from the orbiter incorrectly. For a full report on this phenomena, please see The Planetary Society's article about Spirit from 2006 where this image is also discussed.
Clicking on this image will take you to the High Resolution Image from NASA.