San Juan Islands (săn wän) [key], archipelago of 172 islands constituting San Juan co., NW Wash., E of Vancouver Island. The islands were visited and named c.1790 by Spanish explorers. The islands were the subject of the San Juan Boundary Dispute between Great Britain and the United States; their ownership was decided in 1872. San Juan, Orcas, and Lopez islands are the largest of the group. The San Juan Islands National Monument, est. 2013, encompasses the Bureau of Land Management lands on the islands and includes light stations, a variety of habitats, and many small islands and rocks. San Juan Island National Historical Park on San Juan Island marks the events from 1853 to 1872 involving the boundary dispute (see National Parks and Monuments, table). Other portions of the islands are state parklands.