But now anthropologists from Wheaton College in Illinois and the Field Museum in Chicago have discovered how the ancient Maya produced this pigment and the role it played in important rituals at a famous Maya site in Mexico's Yucatan peninsula.

"Maya blue has long been of interest to scholars, both archaeologists and chemists," Gary Feinman, curator of anthropology at the Field Museum who worked on the study, said in a telephone interview.

"The interest in Maya blue stems from the fact that it is a very durable pigment -- more durable than most natural dyes and pigments. It also stems from the fact it wasn't immediately obvious how it was made and what the key ingredients were."

The pigment resists age, acid, weathering, biodegradation and modern chemical solvents. Previous research had identified two ingredients as extract from the leaves of the indigo plant and an unusual white clay mineral called palygorskite.

These researchers did microscopic analysis on material found in a three-footed pottery bowl in the museum's collection dating from A.D. 1400 that had been used as an incense burner.

RAIN GOD CHAAK

A century ago, the bowl was dredged from the Sacred Cenote, a natural sinkhole well, at Chichen Itza, a key urban centre late in Maya history. They found that copal, a tree resin burned as incense, also was part of the Maya blue concoction.

And they concluded that the pigment was made by mixing the ingredients over low heat in rituals performed on the edge of the sinkhole. That was bad news for human sacrificial victims.

Feinman said that human sacrifice was part of rituals appealing to the Maya rain god Chaak -- depicted on some Maya structures with a unique elongated, curled nose -- to deliver rain for crops such as corn.

During the rituals conducted on the edge of the cenote at Chichen Itza, Feinman said, the Maya seem to have produced the pigment and painted items like pottery that would be tossed into the water as offerings to the god.

And, he added, they also would paint people being offered as human sacrifices blue and heave them into the sinkhole. Feinman said about 120 sets of human remains have been dredged from the sinkhole, along with lots of ceremonial objects.

"Adult males may have had their hearts removed before they were dumped in," Feinman said.

Feinman said at the bottom of the cenote, a layer 14 feet (4.3 meters) deep of blue goo has been found, likely composed of pigment that washed off sacrificial victims and objects.

"The Maya used indigo, copal incense and palygorskite for medicinal purposes," said anthropologist Dean Arnold of Wheaton College and the Field Museum, who also worked on the study.

"So, what we have here are three healing elements that were combined with fire during the ritual at the edge of the Sacred Cenote. The result created Maya blue, symbolic of the healing power of water in an agricultural community," Arnold said.

The findings were published in the journal Antiquity.