Under the Temple of the Feathered Serpent in Mexico lay a tunnel not seen by people in more than 1,800 years.
Damp, dark, filled with debris, and deliberately sealed by its creators, the passageway was thought to house the remains of the rulers of the ancient city of Teotihuacan. A robot that recently explored the tunnel found three secret chambers that may shed light on the millennia-old practices of the city, which the Aztecs called the place where gods are born.
Sergio Gomez Chavez is an archaeologist who has been working on the preservation of this city for 33 years. The tanned 52-year-old is the director of the Tlalocan Project, and a researcher for Mexico's National Institute of Anthropology and History.
"The tunnel represents the Underworld. The offerings found correspond with this idea that Meso-America had of an Underworld. Now we are identifying human remains."
The robot was one of two involved in the project put together by a team that included Hong Kong dentist and science enthusiast Dr Ng Tze-chuen.
Ng has previously helped design robots used to explore a secret chamber in the Great Pyramid of Giza, as well as tools to gather soil and rock samples on missions to Mars and the moon.
Ng was in Mexico for the robot's foray into the unknown. "It's exactly like the Pyramid of the Sun, but that's all looted already," he said, referring to the largest pyramid in Teotihuacan. "This is in pristine condition."
The Tlaloc II-TC carried an infrared camera and laser scanner that allowed for 3-D visualisations of the underbelly of the temple, allowing access to parts of the ruins yet to be excavated.
One of the chambers discovered by the robot contained hundreds of yellow orbs, each between 3.8cm and 12.7cm across, covered in what researchers think is yellow jarosite, a rare mineral.
In addition, the walls and floor of two of the chambers were covered in a sparkling powder made of pyrite (an iron sulphide), hematite and magnetite (both forms of iron oxide). The minerals were thought to have been used to represent the sky and deeps waters of the Underworld. The three chambers have yet to be excavated, and have only been viewed through the images beamed back from the robot.
Researchers have yet to understand the significance of the orbs, but Gomez Chavez said the discovery of the artefacts and chambers would allow archaeologists and historians to get a sense of the system of governing, religion and ways of thinking used by the original inhabitants.
The city is believed to have been built around 100BC, and was home to a multi-ethnic population of more than 100,000, which would make it one of the largest in the world at the time. It was mysteriously abandoned around AD700, but would greatly influence the Aztec and Mayan civilisations. Around 30 workers are at the dig site, excavating and cataloguing their finds, much of which will end up on display at the museum in Teotihuacan, and may be loaned to other museums on travelling exhibits.
Ng, the Hong Kong dentist, contributed about US$10,000 to the project through his work, Gomez Chavez said. The entire excavation project is budgeted at about 8 million Mexican pesos (HK$5 million).
"Tze-chuen has been important in determining how the robot could walk, and how the wheels would work and be durable. He's helped determine how they take samples and images," said Gomez Chavez. "We couldn't step on the soil because it's very humid. The Tlaloc I [robot] had difficulty because of the water in the soil." Some of Ng's design ideas helped get around that problem, he said.
Ng has been involved in several high-profile space exploration projects in the past two decades. Ten years ago he designed planetary sampling tools for the British-designed rover Beagle 2 that was dropped on the surface of Mars, but contact was lost before it landed on the planet. Two years ago a Russian probe carrying tools Ng designed to Phobos, one of the planet's moons, veered disastrously off course.
He is making new tools for the ExoMars rover mission scheduled for 2018, a joint project of the European Space Agency and Russia's federal space agency Roscosmos.