A main substance they measure is phosphorus in the soil of the open plaza. The material is present in food, and if any food or animals' blood spilled in a marketplace, the phosphorus would remain in the dirt.

"We found an alignment of high concentrations of phosphorus in the soil," Terry said. The evidence was invisible to the eye, but soil chemistry showed where food had been spread out in a market.

Chunchucmil was an important trade and manufacturing center, and archaeologists are finding unusual artifacts "such as jade in the homes of commoners," he said. "They're not supposed to be there."

But if a commoner were a successful merchant, maybe he could obtain luxury items. "You might trade corn for salt. You might trade fruits and vegetables for pottery." Obsidian — a volcanic glass that can be chipped into blades as sharp as today's surgical instruments — also may have been traded, Terry noted.

Besides phosphorus, scientists are looking for metal residue in the soil, particularly iron. "We found a couple of areas of high levels of iron," he said. Iron was part of the red pigment that the Maya loved, and also is in meat and blood.

It's possible areas high in iron were used for butchering animals in the marketplace. "We need more evidence to tell for sure. Those are studies we hope to do this coming season," Terry said.

Dahlin said the soil had accumulated "something like 40 times the phosphates" compared with other areas.

"It was a marketplace. Certainly food was brought in and spilt ... over a couple of centuries. That's how these phosphates had accumulated."

Phosphorus levels in the soil samples were compared with those in the soil of a modern marketplace in Antigua, Guatemala. According to the report, that open-air marketplace was set up after an earthquake hit in 1976. It's the only market in the region with a dirt floor, as others are on paved surfaces.

High levels of phosphorus were found in the soil around places where vegetables and fruit are sold, the paper says, probably evidence of food that was spilled or dripped.

Dahlin noted that before Terry and his students arrived at Chunchucmil, archaeologists were examining what looked like rocks piled in rows in the plaza.

"They were too small to be houses," he said. "They looked like market stalls."

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