The new photo shows a wall on one side of the 'hole' indicating that it's most likely a pit and not, as speculated, the entrance to an underground cavern. The HiRISE (High Resolution Imaging Science Experiment) collaboration calculates from the shadow of the rim cast onto the wall of the pit that the pit is at least 78 meters (255 feet) deep [source].
[Image: NASA/JPL/University of Arizona]
For more info, see e.g. J.P. Skipper's Mars Anomaly Research, Space.com "Deep Hole found on Mars", or the HiRISE websites.