Sikkim is commemorating 50 years of its integration with India, which took place on 16 May 1975. The Himalayan kingdom was ruled by the Chogyal monarchy from 1642 until 1975. Under the Indo-Sikkim Treaty of 1950 it had been an Indian protectorate enjoying internal autonomy. In a 1975 referendum, about 97% of voters chose to abolish the monarchy and join India, after which the 36th Constitutional Amendment Act, 1975 made Sikkim the 22nd state of the Indian Union. Located in the eastern Himalayas, Sikkim also became the world's first fully organic state in 2016.