The town of Gede (aka Gedi), a few kilometres inland from Watamu, was inhabited from around the 12th or 13th century until approximately 600 years ago. The reason for abandonment is unknown, although speculation is either that the sea receded, or that a local river changed course, leaving the town (which has many deep wells) without water. Information on the 'net is hard to find, but the guidebooks to the area have more details. It's a beautiful site, and well worth visiting.