Perdido Key is a 24-kilometre-long (15 mi) barrier island on the north coast of the Gulf of Mexico. The island extends from Pensacola Pass on the east to Perdido Pass on the west. Most of the island is in Florida, with the western end of the island in Alabama. Santa Rosa Island is to the east of the island, and Alabama Point is to the west. Perdido Key was attached to the mainland until the 1940s when a canal separated it from the mainland. The eastern part of the island forms part of the Gulf Islands National Seashore, which includes the Perdido Key Historic District, the site of three shore batteries that once defended Pensacola Bay. The rest of the island, with the exception of the Perdido Key State Park, has been developed as a resort and residential community. As a barrier island, Perdidio Key is subject to changes due to wave action and longshore currents, particularly from hurricanes. The endangered Perdido Key beach mouse is endemic to the island.