Simpson Bay Lagoon is one of the largest inland lagoons in the West Indies of the Caribbean. It is located on the island of Saint Martin/Sint Maarten. The border between the French and Dutch halves of the island runs across the centre of the lagoon. There are two small islands that lie in the lagoon: the larger, Grand Ilet (also known as Explorer's Island) to the north, is within the French region of Saint-Martin; the smaller, Little Key, to the south, is on the Dutch Sint Maarten side.