Inauguration of the El Atazar dam Historical milestone · El Atazar, Sierra Norte, Madrid Work began in October 1965 and the great arch dam on the Lozoya was opened in April 1972, after its cost had multiplied owing to the geotechnical problems of the slates. Its reservoir is today the largest in the Community of Madrid. 1965-1972 Photo: Dirección General de Turismo. Consejería de Economía e Innovación Tecnológica. Comunidad de Madrid (CC BY 3.0 es) What to see and do in El Atazar