Excitement was in the air when someone "found Atlantis" off the cost of Africa recently. According to Google, what looks suspiciously similar to under-water roads is simply an anomaly caused by the ships that collected the data.[image from Google Maps/Earth]
I'm sure people would have come to the same conclusion Google did if they studied the images seen on Google Earth -- city blocks appear to be anywhere from 5 to 10 miles long, and the roads themselves are almost a mile wide.
There have been some important discoveries made using Google Earth, but it's going to be difficult to make many under-water discoveries given the resolution that is currently available.