What lies beneath: Muon detectors are unleashing the secrets of what lies beneath Egyption and Mayan pyramids, as well as other ancient archaeological ruins.
Credit: Photolibrary
IN THE DENSE JUNGLES of northwestern Belize, the sound of metal hitting rock startles flocks of tropical birds and troops of black howler monkeys. Archaeologist Norman Hammond and his team stop digging. They have hit a stone wall three metres below ground; beyond it is a royal tomb. After almost 100 test excavations at La Milpa, an ancient Mayan city that once housed about 50,000 people, Hammond has unearthed something big.
Within the tomb lies the remains of a man wearing a jade pendant in the form of a vulture's head. He is thought to be either the Maya king Bird Jaguar, the fifth-century ruler of La Milpa, or one of his successors. And finding him was an incredible stroke of luck.
Hammond, an expert on pre-Columbian archaeology, made his famous discovery more than 10 years ago. But repeated efforts to find other tombs at the site have come up empty-handed. Lacking an ancient map of La Milpa or blueprints for its five pyramid mounds and buried plaza, archaeologists rely mostly on instinct. But they run the risk of piercing priceless relics with their shovels or digging fruitlessly for years.
The ground beneath the site "could be Swiss cheese, for all we know," riddled with burial chambers, tunnels and hidden entrances, says Hammond, who is based at Boston University in Massachusetts, USA.
Now researchers hope to find those hidden spaces with a little help from a most unexpected quarter: the particle physics community.
The key to the new approach is the muon, a hefty cousin of the electron that's created when cosmic rays hit the atmosphere. Muons pass harmlessly through people and buildings; around 600 of them have flown through your body since you started reading this article. They fascinate scientists because they're one of the few high-energy particles raining down from the sky that can be examined for clues to the nature of the cosmos. But it's the ability of the muon to penetrate deep into rock and water that has archaeologists excited.
Muons travelling through rock or other dense material will slow and eventually stop, while those flying through empty spaces keep going at full speed. The idea is to catch the muons after they've passed through an archaeological site and measure their energies and trajectories. This allows researchers to reconstruct their paths and compile a three-dimensional (3-D) image that reveals hidden chambers or other voids.
If you think of the archaeologists as like surgeons probing a patient, particle physicists are like radiologists whose non-invasive X-rays reveal what's going on inside the body. Just as X-rays leave patients unscarred, muons offer a way to explore ancient ruins without disturbing them.
A team led by Arturo Menchaca Rocha, head of the physics institute of the National Autonomous University of Mexico (UNAM), in Mexico City, plans to place a muon detector beneath the Pyramid of the Sun in Teotihuacán, northeast of the capital.
Meanwhile, particle physicist Roy Schwitters of the University of Texas at Austin, USA, is making plans to install muon detectors in wells dug on opposite sides of a mound at La Milpa where Bird Jaguar was found. The detectors will quietly gather muons for about a year, slowly building a picture of the interior of each pyramid.
