When we left for the tour, Mr. López rode ahead in a white pickup and I followed in a white van filled with borough workers and a reporter and photographer from a Mexico City newspaper. We made our first stop at the orange museum built in the shape of a pyramid on the Hill of the Star’s northern flanks, the New Fire Museum.  After a brief tour of the museum’s mural, dioramas depicting the New Fire ceremony and explanations of the 52-year calendar cycle, Mr. López led us up the road to a fenced-off cave near the summit called the Cave of the Devil.  “It’s filled in with dirt,” Mr. López explained, pointing out a metal post supporting one of four security cameras on the hill.  Across the road from the Cave of the Devil, a narrow stairway led to the ruins, where a broad pyramid base provides views of downtown skyscrapers, majestic boulevards and planes landing at the airport. If you’re lucky, and when I went I wasn’t, wind will have cleansed the capital of pollution and you will be able to see the big volcanoes of Popocatépetl and Iztaccíhuatl in the distance.  Mr. López bounded back down the trail, leading us through another cave with rooms once inhabited by pre-Hispanic residents and past petroglyphs of faces and stars carved in the rock.  At the bottom of the hill, in a small park near a major road, Mr. López and his helpers distributed helmets, kneepads and surgical masks. Then we walked through an opening in a fence and stepped down over volcanic rocks and bags reeking of trash into an opening that leads beneath Mexico City. The darkness of caverns swallowed the beam from Mr. López’s flashlight as he explained that the Aztecs believed such caves provided passage to Mictlan, the underworld. He said local geologists are still mapping the volcano’s labyrinth of more than 100 caves.  “We don’t know how deep it goes,” he said.  A few days later, I met Jesús Sánchez, the archaeologist who studied the Hill of the Star from 2002 through 2008 for Mexico’s National Institute of Anthropology and History, at a restaurant in the south of Mexico City.  The New Fire ceremony is only part of what distinguished the Hill of the Star, Mr. Sánchez, wearing a blue Tennessee Titans sweatshirt, told me over a beer. The volcano formed a peninsula that narrowed the passage along the surrounding lakes, and it divided sources of salt water from fresh water. That made controlling it essential for both military dominance and agricultural production.  “Whoever occupied El Cerro practically dominated the whole Valley of Mexico,” he said. Mr. Sánchez said he and his crew partly excavated a pyramid built in the year 500 by the Teotihuacan, who predated the arrival of the Aztecs. Work on those ruins remains unfinished for lack of funds, and the annual passion play that attracts more than a million people to the Hill of the Star each Holy Week has complicated efforts to preserve them. The organizers of that event have traditionally reenacted the crucifixion of Christ right on top of the ruins without knowing it.  “We are very afraid that archaeological evidence that still survives on El Cerro and that is so important for Mexican history will be lost within 10 years,” he said. “To lose El Cerro is to lose the foundation of our Mexican identity.”  I went back to the Hill of the Star again on public transportation on a Sunday morning — 23 minutes on the metro and then 10 minutes on a sputtering bus through graffiti-covered streets — but the most memorable trip came in the darkness, when I returned a week later.  My driver cruised through stoplights at 5:45 a.m. and floated across lanes on San Antonio Abad, a major thoroughfare with a metro line running down the middle built on top of the Aztec road across Lake Texcoco. We navigated through narrow, graffiti-lined alleyways by cellphone until we found the museum. We parked in front of the Cave of the Devil.  A storm that had scrubbed the sky of pollution had passed, and we looked out over the lights of the city filling the lowlands as the lakes once had. The volcanoes took shape against the horizon growing blue and orange. When the sun became molten over the hills, I noticed a man at the corner of the pyramid base kneeling in prayer. A group of four women in jogging clothes raised their arms to the light.  I thought about all the people who had been so aware that the world was fragile and that it might end, as they looked up from the lake toward the Hill of the Star.  I thought about what it must have been like to see that first spark of light in the darkness.  A version of this article appears in print on April 17, 2016, on page TR9 of the New York edition with the headline: Layers of History on a Sacred Volcano. Order Reprints| Today's Paper|Subscribe
The Hill of the Star, once one of the most important Aztec religious sites, has been lost to time. But a visit there can still be transportive.