Teotihuacan is an ancient Mesoamerican city located in the sub-valley of Mexico, known for its massive pyramids and the long 'Avenue of the Dead.' At its peak in the first half of the 1st millennium AD, it was the largest city in the pre-Columbian Americas, with an estimated population of over 125,000. The city's most iconic structures are the Pyramid of the Sun—the third largest pyramid in the world—and the Pyramid of the Moon. Unlike the Aztecs, who later occupied and named the site (meaning 'The Place Where Gods Were Created'), the original builders' identity remains a mystery. The city was meticulously planned on a grid system, suggesting advanced knowledge of mathematics and astronomy. Its influence on later civilizations, including the Maya and Aztecs, was profound, particularly in terms of religious iconography and urban planning.