low-lying island on the north shore of the Thames estuary, Castle Point borough, administrative and historic county of Essex, England. It is connected to the mainland by a bridge at South Benfleet. The island's marine defenses were first constructed by a Dutch engineer in 1623, but it has continued to be vulnerable to exceptional tidal flooding. It is the site of petroleum-refining development and has extensive storage facilities for imported liquefied natural gas. Pop. (2001) 37,479.