On another note, why are we discovering this just now? I was under the amateur impression that the entire surface of Mars was scanned and imaged by NASA at some point. But again, I may have interpreted that incorrectly.
It's easy to forget just how large Mars is, as a comparison, if we scaled Mars up to the size of Earth, the hole would only end up being about 120 meters, or about %30 larger than an American football field. And when it's a feature that we pretty much need human eyes on to determine it's significance, it's easy to imagine how we've missed it. Just imagine trying to find a random football field sized thing in Google Earth, somewhere on the planet.
And to make the comparison even more valid, Mars isn't covered in water, and has a very comparable amount of land surface area as Earth does. in that case, it makes the hole only about 36 meters when scaled up to the surface are of Earth, which is likely what you'd be searching for in Google Earth.
Not to mention that I have no idea to what resolution Mars was scanned and imaged at, considering imagery I see of Earth often, the mentioned scans could easily have pixels larger then the entire size of that feature.
Hence it's easy to imagine that many more interesting features of similar size exist on the martian surface, sitting there in plain view, waiting to be discovered.
It's probably safe to assume we don't have imagery this good for the entire surface of Mars.
Edit: It seems likely that these new images are captured with this equipment: http://en.wikipedia.org/wiki/HiRISE
Or did you mean StreetView?
Since 2006, there's been a high-resolution camera platform orbiting Mars (http://en.wikipedia.org/wiki/HiRISE), and it's doing this high-res imaging, at up to 30cm/pixel (!), for the first time. Though Wikipedia says that as of 2010, it's only mapped 1% of Mars's surface to that degree.