Manaus is the capital city of the state of Amazonas in northern Brazil. It is unique because it is situated in the middle of the Amazon rainforest at the confluence of the Negro and Solimões rivers. This confluence is known as the 'Meeting of Waters,' where the dark-colored Negro River and the sandy-colored Amazon River (Solimões) flow side-by-side for several kilometers without mixing due to differences in temperature, speed, and water density. Manaus was once the world's 'Rubber Capital' during the 19th-century rubber boom. Despite its remote location deep within the jungle, it is a major metropolitan area with over 2 million residents, accessible primarily by boat or airplane.