"This material is formed by the oxidation of pyrite, which is a metallic ore," Gómez Chávez said. "It means that in pre-hispanic times they appeared as if they were metallic spheres. There are hundreds of these in the south chamber."
According to George Cowgill, professor emeritus at Arizona State University and the author of several publications on Teotihuacan, the spheres are a fascinating find.
"Pyrite was certainly used by the Teotihuacanos and other ancient Mesoamerican societies," Cowgill told Discovery News. "Originally the spheres would have shown brilliantly. They are indeed unique, but I have no idea what they mean."
Even the walls and ceiling of both chambers were covered with a mineral powder composed of magnetite, pyrite and hematite which provided a special brightness to the place.
"We believe that high-ranking people, priests or even rulers, went down to the tunnel to perform rituals," Gómez Chávez said.
Indeed the archaeologists found many offerings, including pottery and wooden masks covered with inlaid rock crystal, jade and quartz -- all dating from around 100 A.D.
Gómez Chávez and his team now look forward to the next phase of the project -- exploring the last part of the tunnel and three chambers which archaeologists have seen through the robot cameras.
"The tunnel is in pristine condition, untouched for almost two millennia," said Ng "TC" Tze Chuen, an independent researcher who worked on the design of the Tláloc II-TC robot. "Can you can imagine what can be found inside?"
Ng, who helped create the Djedi robot that explored Egypt's Great Pyramid in 2010, believes the Mexican tunnel might lead to one of the most significant archaeological finds in Teotihuacan.
"The results are very encouraging indeed," he said.
According to Gómez Chávez, the tunnel was sealed twice by the Teotihuacan people. Thick walls, erected to block access, were demolished about 1,800 years ago in order to deposit something very important in the central chamber at the end of the tunnel.
"Maybe in this place," Gómez Chávez said, "we will find the remains of those who ruled Teotihuacan."