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Many mysteries surround the ancient Mexican city of Teotihuacan. One mystery is especially difficult to solve. What is the meaning of the symbols that are on its paintings and statues?

Earlier this month, the area's pyramids reopened to visitors as coronavirus restrictions eased. But the most interesting neighborhood remains off-limits to tourists.

The neighborhood has a patio floor with rare painted symbols, known as glyphs. Their discovery, in the 1990s, led a growing number of experts to question the long-held belief that the city did not have a writing system. These experts hope to one day understand the meaning of the glyphs - just as other scholars have done with ancient Mayan and Egyptian writing.

Teotihuacan is about 50 kilometers outside the Mexican capital, Mexico City.

Teotihuacan was possibly founded more than 2,000 years ago. It was an active city for over six centuries.

At one time, it was the largest city in the Americas and home to at least 100,000 people.

Yet much is unknown about its occupants and their civilization. It is unclear what language they spoke. It is also unclear if they developed a system of writing like that of the Aztecs. The Aztecs controlled the same area about 800 years later.

Experts have debated several theories about the glyphs. They say the glyphs may have been used to teach writing or may have been the names of places.

Art historian Tatiana Valdez wrote a book about the glyphs of Teotihuacan. She says the patio's 42 glyphs are the longest text ever found at the city's ruins.

Valdez added that more than 300 of Teotihuacan's glyphs have been tentatively identified.

Many ancient Mexican codices - paper books covered in ancient writing - were ordered to be burned in colonial times by Catholic officials. Only about 10 of the codices still exist.

Valdez believes that such books were also part of Teotihuacan's written tradition.

Walking around La Ventilla, where you can find the patio, is like exploring an ancient neighborhood. It has religious centers, stores, houses, and beautiful paintings.

Mexico's government-operated National Institute of Anthropology and History says more work is still needed to be able to open the area to visitors.

Pictures and images of the city's most recently confirmed glyph will likely be published in a paper next year.

It is the kind of find expected by the University of Michigan's Joyce Marcus, who has argued that writing was absent in the city.

"So far, we have not seen the long texts," she told Associated Press reporters in an email.

A painting discovered in the 1960s in Teotihuacan shows what appears to be a religious leader holding a book. The picture was a "hugely important" discovery, said Christophe Helmke of the University of Copenhagen. He is a leading expert on the city's writing system.

Helmke added that writing in Teotihuacan was probably mostly limited to its books.

David Stuart of the University of Texas said it is difficult to understand Teotihuacan's glyphs because researchers are not sure what language was spoken there.

Stuart has been a leader in decoding ancient Mayan writing.

"It's true that many still say that Teotihuacan had no writing system," he said. "But, in fact, it's there."

I'm John Russell.

David Alire Garcia reported on this story for Reuters news agency. John Russell adapted it for VOA Learning English. George Grow was the editor.