The Aztecs were an advanced civilization that thrived in central Mexico in the 14th, 15th, and 16th centuries. Their capital city, Tenochtitlan, was built on an island in the middle of Lake Texcoco, in the Valley of Mexico. Tenochtitlan was the largest city in the Americas at the time, with a population of over 200,000 people. The Aztecs also had a vast empire that stretched from the Pacific Ocean to the Gulf of Mexico and included parts of what is now Mexico, Guatemala, Belize, El Salvador, and Honduras.