Archaeologists are to open a long-sealed cave under a Mexican pyramid in the hope that it will unlock the mystery of one of ancient civilization’s greatest cities.

With its soaring stone pyramids and geometric temples, Teotihuacan was once the biggest city in the Americas and possibly the world.

However, experts have never been able to say with certainty who built it and

why it was suddenly abandoned.

An international team of experts believes the answer may lie under the Pyramid of the Sun, the center point of the vast ruined city 25 miles outside Mexico City.

At the end of this month, they are to investigate a man-made tunnel and cave system underneath the pyramid — the third biggest in the world — to test theories that it was used for rituals including human sacrifice.

“We think it had a ritual purpose. Offerings were placed at the very end of the tunnel as part of the pyramid’s construction process,” Teotihuacan’s director of archaeology, Alejandro Sarabia, said.

He will lead a team of Mexican, American, and Japanese experts into a

295-foot-long, 8-foot-high tunnel some 20 feet below the pyramid.

“We want to find out why the Teotihuacan people sealed it and when,” Mr. Sarabia said. “Excavating the cave could give us some clues about what happened at Teotihuacan, about the fate of the city.”

At its zenith between 150 C.E. and 450 C.E., Teotihuacan was home to up to 200,000 people of various ethnic origins and thought to have been larger than any European city at the time, including Rome.

But, sometime in the seventh or eighth century, it was set ablaze — possibly as the result of an insurrection — and abandoned.