A Mystery For Millennia, This Ancient Pyramid May Crumble Soon

The Sun Pyramid, built by ancient pre-Aztecs around 100 B.C., is in trouble. A bad reconstruction job a century ago may be causing one side of the pyramid to dry out, and some say it could crumble.

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One of the largest pyramids in the world is in trouble: Mexico's Pyramid of the Sun. It's located just outside Mexico City and is a popular tourist attraction. A local physicist says part of the 2,000 year old structure appears to be drying out and local news headlines have spread fear that the revered ruins are in danger of collapsing. As NPR's Carrie Kahn reports, an archeological reconstruction fix done 100 years ago may have put the ancient pyramid in peril.

CARRIE KAHN, BYLINE: Standing at the foot of the massive Pyramid of the Sun, tourist Andres Sovince, of Slovenia, and his friend Dan Laffoley, of England, are in awe.

ANDRES SOVINCE: It's fantastic. This is such an astonishing place.

DAN LAFFOLEY: For me, it's the equivalent as when you're in Beijing and you've got the Forbidden City and you walk through and it's in front of you and you just go, wow. And that's the same kind of feel you get here.

KAHN: The Pyramid of the Sun was built around 100 B.C., says archeologist Victor Castaneda Leanyo.

VICTOR CASTANEDA LEANYO: The construction lasts about 100 years, 150 years.

KAHN: It took them 150 years to build this?

CASTANEDA: Yes.

KAHN: Castaneda says no one really knows why the ancient inhabitants built this magnificent pyramid. The Aztecs who came about 600 years later used it for their religious purposes, but the original builder's plan is still a mystery. No one has ever seen what's deep inside until now.

ARTURO MENCHACA: It's the longest experiment I've ever been.

KAHN: Dr. Arturo Menchaca is a nuclear physicist at Mexico's Autonomous University. As a postdoctoral student at UC Berkeley in the 1970s, he had a chance meeting with another physicist, Nobel Prize-winner Luis Alvarez. Alvarez had had successfully scanned the inside of one of the pyramids in Egypt using a muon detector. Muons are cosmic particles heavy enough to penetrate rock.

If more muons are detected in one spot than another in a pyramid, it most likely signals a void in the structure or a chamber. Menchaca dreamt of scanning Mexico's Sun Pyramid for chambers. He finally started in 2000, but first had to get the right permits and, of course, assemble his own machine.

MENCHACA: Those detectors are not sold in the supermarket. You have to build them.

KAHN: It took him 10 years to build it and Menchaca did find a difference in muon readings inside the pyramid, a huge difference. He believes the north side is more dense than the south side. The readings are too big to point to a huge chamber inside, but Menchaca says the south side where the sun hits the hardest appears to be drying out. Local reporters took that to mean that the south wall of the pyramid could collapse. Menchaca says that's unlikely any time soon.

Undaunted by reports of impending collapse, I hike to the top with archeologist Castaneda. About five more steps and we're up to the top. Ooh. Here we go. We made it. You made it.

CASTANEDA: We did.

KAHN: What a view, huh? He says the culprit that could've lead to the pyramid's drying south side was a government-sanctioned reconstruction 100 years ago. Archeologists spent five years covering the dirt pyramid with a layer of cement and stone. Rain can no longer penetrate the pyramid like it used to. Castaneda says he hopes the government will fund a proper fix that involves removing part of the concrete cover to not only let in the rain, but also archeologists.

CASTANEDA: Just under this concrete layer, there is virgin, fresh area for archeology. That's incredible.

KAHN: You have a very big smile on your face. You want to do it.

CASTANEDA: Yes, I want to do it.

KAHN: Dr. Menchaca's research will be completed this year and then it's up to the government how to proceed and fix the structural problems. And while the pyramid's drying out grabbed all the recent headlines, the original question was lost in the uproar. Are there chambers inside the Grand Pyramid of the Sun? Could there be the body of a king or other ruler inside? Dr. Menchaca won't say.

MENCHACA: We are not revealing any other detail of our data until we have a paper published. I cannot comment.

KAHN: Stay tuned. Carrie Kahn, NPR News, Mexico City.

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