The Pyramid of the Sun may fall apart. One side is dry while another side is wet, which could lead to the pyramid's collapse unless a fix can be found.
Between the 1st and 7th centuries, Mexico's Pyramid of the Sun was at the heart of the largest city in the Americas. Now known as Teotihuacan, the lost city had a population of more than 125,000, making it one of the biggest in the world. The pyramid itself is among the largest on the planet. Its exterior is covered with 3 million tonnes of volcanic rock, but the interior is a mound of earth.
From 2010 to 2013, Arturo Menchaca of the National Autonomous University of Mexico (UNAM) in Mexico City and colleagues studied the interior of the pyramid using muons. These sub-atomic particles can pass through most materials, but are deflected when they hit denser ones. That means more muons reach the other side if an object has an internal cavity, filled with less dense air. So by tracking the paths of muons through the pyramid, Menchaca could create a 3D representation of its insides.
To do this, his team placed muon detectors under the centre of the pyramid, in a tunnel that runs beneath its base. The muons originate in space as cosmic rays, which break up into smaller particles when they pass through Earth's atmosphere.
The team was looking for internal chambers, but none was apparent. In contrast, the nearby Pyramid of the Moon contains royal tombs.
Instead they found a problem: the density of the earth in the pyramid is at least 20 per cent lower on one side than the other. "The pyramid is at risk of collapsing if something isn't done," says Menchaca. He presented his results at a conference on Teotihuacan at UNAM last month.
Menchaca believes the difference is caused by the south side drying out. He compares the pyramid to a sandcastle on a beach. "I can use slightly moist sand to make a sandcastle," he says. "If I leave it exposed to the sun and touch it when it is dry, then it crumbles."
The pyramid is "not going to collapse tomorrow", Menchaca says. "But it is the same phenomenon we observe in the subsoil of Mexico City." Mexico's capital is built on a dried-out lake, and every year the city sinks by tens of centimetres as water is extracted from aquifers beneath it.
Opinion is divided on how to save the pyramid. Menchaca suggests wetting the dry side.
But the real problem may be excess water, not dryness, says Alejandro Sarabia, the site director at Teotihuacan. "Decades ago, cement was added between the covering stones. This added stability and hindered the growth of vegetation," he says. "On the other hand, it prevents evaporation of damp created by water seeping through gaps."
Sarabia says archaeologists are now replacing the cement with more suitable materials like river sand.
This article appeared in print under the headline "Mexico's great pyramid under threat of collapse"