Archaeological sites in Mexico City have street addresses, not GPS coordinates, as sites tend to elsewhere. At this particular address, behind the green door, next to the Calmécac, archaeologists uncovered the Temple of Ehécatl-Quetzalcoatl, a structure dating from about 1450. The temple, whose distinctive, round shape was described by Spanish priest Bernardino de Sahagún, was located about 80 feet north of where Spanish colonial maps had originally shown it to be. Ehécatl was a wind god sometimes depicted as a version of Quetzalcoatl, the feathered serpent who had already been worshipped in central Mexico for more than 1,000 years by the time Tenochtitlan was founded in 1325. In fact, snake imagery abounded at the temple in antiquity. Spanish chroniclers described the building as having a conical roof made of straw, resembling a coiled snake. To enter, worshippers passed through a stone arch carved to resemble a snake’s mouth, complete with fangs. The Spaniards associated serpents with the Garden of Eden story, regarding the reptiles as evil, and usually destroyed snake images wherever they saw them. But, if the temple’s snake arch wasn’t destroyed by the Spaniards, it may still lie buried beneath a row of buildings behind the Metropolitan Cathedral, awaiting discovery.

Excavation has shown that the Guatemala Street temple was bordered by a long outer wall, which the modern street directly above it follows exactly. This is no coincidence, but rather evidence that the Spaniards stuck closely to the original Aztec urban grid when they built their own city on the ruins of Tenochtitlan. Modern avenues also run along the same lines as causeways that once connected the ancient island city to the mainland.

In 1985, an earthquake measuring 8.1 on the Richter scale killed some 10,000 people and destroyed or compromised thousands of buildings in Mexico City. Some of those buildings happened to have been standing over Aztec civic and holy sites. More than two decades later, after workers demolished a building rendered structurally unsound by the quake, archaeologists dug down and found the ruins of an elite school near the Templo Mayor. Known as the Calmécac, which in the Nahuatl language spoken by the Aztecs means “school,” the complex was where Aztec nobility sent their children to be trained in war and worship. “The school’s proximity to the Templo Mayor shows the elite’s concern for educating young men for power,” says Harvard historian David Carrasco. The emperor Moctezuma II himself was a graduate.

An enormous structure in antiquity, even larger than the Templo Mayor, the school had a courtyard whose roof was adorned with a row of spiral ornaments representing snails, which were associated with the rain god Tlaloc. Spanish colonial-era drawings had suggested these adornments were small, even dainty, decorative touches. But when archaeologists discovered them, the ornaments actually stood a monumental eight feet tall and must have been visible from all over Tenochtitlan. Of the seven found by archaeologist Raúl Barrera, all had been removed in antiquity from their rooftop perches and laid below a floor. By the time the Spaniards arrived, they had been replaced with similar ornaments that the Spaniards later destroyed, of which no traces have been found. Since their rediscovery, the Calmécac roof ornaments have become one of the most distinctive motifs of ancient Mexico. Excavation at the Calmécac proved difficult. Eighteen feet beneath the city, the site continually flooded and had to have water pumped out, a problem that speaks to the city’s unusual geography. Tenochtitlan was built on a group of marshy islands in the center of Lake Tezcoco. These were gradually filled in with lines of tree trunks and soil using an ancient land-reclamation technique similar to that employed in Tenochtitlan’s contemporary city, Venice. As in Venice, canals crisscrossed the city. Archaeologists have found traces of some of them, as well as a pier that jutted into the lake in antiquity. Lake Tezcoco has been almost completely filled in over the centuries, but the soil underneath the city remains porous and damp, “like gelatin,” says archaeologist Eduardo Matos Moctezuma. Although the city has been gradually settling at a rate of up to 20 feet per century into the lake bed, not so the Templo Mayor, which was built on sturdy landfill. It is therefore sinking at a much slower pace, causing it to gradually “rise” relative to its surroundings such that it will, eventually, regain the 150-foot height it had in antiquity.

Once the remains of the Calmécac were stabilized, archaeologists discovered walls and wide staircases, some with ancient footprints still in their stucco surfaces. They also uncovered dozens of artifacts that hint at student life in A.D. 1500, including well-worn ceramic plates, a clay spoon, and flint and obsidian knives that probably had both practical and ceremonial uses. PAU director Raúl Barrera has excavated only a small corner of the ancient school because most of it remains beneath busy Donceles Street and its taco stands and cantinas. Digging any further would endanger those buildings’ foundations, he explains, “and then, instead of us excavating, someone would have to come excavate us.”

A half-hour walk north of the Templo Mayor, Tlatelolco was a rival Aztec city until it was absorbed into Tenochtitlan in 1473. Recent excavations have shown that Tlatelolco’s ceremonial complex was once almost as large and impressive as that of the main Aztec capital, although at the time of the Spanish conquest, the city was known mostly for its thriving market. Tlatelolco was the final redoubt of the Aztec emperor Cuauhtémoc before he was captured by Cortés in August 1521. Cortés later released Cuauhtémoc and allowed him to continue to rule but, fearing a conspiracy, had him executed in 1525. He was the last Aztec ruler.

Just over a decade ago, archaeologists made an intriguing discovery at Tlatelolco. Beneath a colonial church erected over Aztec foundations, they found a seven-foot-deep, 26-foot-wide basin that had been built on Cuauhtémoc’s orders. Known as a caja de agua, or “water box,” the basin was fed with water from Chapultepec Hill, some four miles away. A system of aqueducts ensured the city’s supply of potable water, as lake water was not suitable for drinking. This cistern was, perhaps, the last example of Aztec civic construction.

On the basin’s walls, archaeologists discovered murals, once brightly colored but now faded with age. Painted just as the Spaniards were consolidating their power, the frescoes are a unique hybrid of Aztec and Spanish themes. They show scenes of canoes on a lake, people fishing, ducks, reeds, water lilies, frogs, herons, and jaguars. In one scene, a fisherman casts a net while, at his feet, a coiled snake tries to eat a frog. Snakes and frogs had deep symbolic associations for the Aztecs, and were depicted in the basin in a naturalistic, European manner. “These murals were painted at the moment of the conquest. In a way, they show the encounter of the European and Mexican cultures,” says archaeologist Salvador Guilliem. Tlatelolco, where the Aztec world made its last stand, was thus also the scene of one of the initial artistic expressions of modern Mexico.