5-Nights Monarch Butterfly Photo Expedition — CALL

From the Alameda, a leafy center of activity since Aztec times, to the Zona Rosa, a chic shopping neighborhood, Mexico City offers endless options to urban adventurers.

Founded by the Aztecs as Tenochtitlán in 1325, Mexico City is both the oldest and the highest (7,349 ft) metropolis on the North American continent. And with nearly 24 million inhabitants, it is the most populous city in the world. It is Mexico's cultural, political, and financial core -- braving the 21st century but clinging to its deeply entrenched Aztec heritage.

You only need to stand in the center of the Plaza of Three Cultures to visually comprehend the undisputed significance of this city. Here, the remains of an Aztec pyramid, a colonial church, and a towering modern office building face one another, a testament to the city's prominence in ancient and contemporary history. Located at the heart of the Americas, Mexico City has been a center of life and commerce for more than 2,000 years. The Teotihuacán, Toltec, Aztec, and European conquistadors all contributed to the city's fascinating evolution, art, and heritage. Although residents refer to their city as simply México (meh-hee-koh), its multitude of ancient ruins, colonial masterpieces, and modern architecture has prompted others to call it "The City of Palaces."

The central downtown area resembles a European city, dominated by ornate buildings and broad boulevards, and interspersed with public art, parks, and gardens. This sprawling city is thoroughly modern and, in places, unsightly and chaotic, but it never strays far from its historical roots. In the center are the partially excavated ruins of the main Aztec temple; pyramids rise just beyond the city.