It is known as Pheasant Island. Less than an acre in size, it was a regular royal meeting place several centuries ago, typically for bridal exchanges. In 1615, Louis XIII of France and Philip IV of Spain first met their wives there (after already having married them). Louis XIV and Charles II would also first meet their brides there.
The island was considered so historically significant that in 1659, a treaty was issued (simply called The Treaty of 1659) that established it as a border arrangement called a condominium. A condominium is a territory jointly run by multiple countries, and is usually on the border between those countries.
Condominiums are rare because they require a heavy amount of cooperation, and thus rarely last long. Pheasant Island is actually the oldest surviving condominium. It is unique in that it is the only condominium known that is traded off, rather than shared at the same time.
Every six months, Spain and France trade control of the island. Because of this, the island has passed ownership more than 700 times in the last five centuries.