Tenochtitlan, also known as Mexico-Tenochtitlan, was a large Mexican altepetl in what is now the historic center of Mexico City. The exact date of the founding of the city is unclear, but the date 13 March 1325 was chosen in 1925 to celebrate the 600th anniversary of the city. The city was built on an island in what was then Lake Texcoco in the Valley of Mexico.