From
http://www.bermudatourism.com/default.asp?page=what_to_do.html:
Harrington Sound is an enclosed saltwater lagoon, with a narrow channel at Flatts Inlet (at the Bridge), allowing a strong tidal current, which changes direction every six hours. However, only half of the tidal flow to the Sound comes through this inlet – the rest comes through a myriad of underground cave systems which surround the Sound. This has resulted in Bermuda’s deepest inshore water area.