A circular earthwork enclosure can be seen on aerial photographs taken in 1947. The remains of this earthwork are situated on the north-eastern end of Hoo Island. The enclosure is defined by an earthwork embankment 70m in diameter with an outer ditch 6m wide. Although the embankment appears complete in 1947 all but the south-western part of the outer ditch had eroded away by that date. Erosion has continued since 1947 and by 2007 the northern and north-eastern section of the embankment had also been lost to the sea. The photographs show two breaks in the ditch at the south west and south-east; map evidence shows a third to the north. These provided access and the remains of a causeway leaving the south-western entrance could still be seen in 1947.This earthwork may be the remains of an early 18th century airing ground used when quarantine was in force. Quarantine involved a period of detention for a ship and those on board and the airing of the ship¿s cargo. Quarantine was in force between 1709 and 1714 due to an outbreak of bubonic plague in the Baltic. London bound ships were held in Stangate Creek on the south side of the Medway. Airing grounds were originally proposed for the Isle of Grain but were eventually put on Hoo Island despite concerns expressed by Rochester and surrounding towns. Up to three ships could unload on Hoo Island and airing sheds were built. Hoo Island was used again as a result of an outbreak of bubonic plague at Marseilles in 1720. Further sheds were built and the governor of Upnor castle, a little downstream from the island, was ordered to `watch¿ the sheds being erected. The Marseilles Crisis ended in 1723 and this may mark the final use of Hoo Island for airing quarantined goods. This was mapped from aerial photographs as part of the English Heritage: Hoo Peninsula Landscape Project.