In the shadows of this modern city's gleaming towers, under the remains of a blighted apartment block, archaeologists digging through the rubble of centuries have uncovered a mysterious circle in stone.

The circle, formed of dozens of holes bored into the limestone bedrock with rudimentary tools and located just a few steps from the mouth of the Miami River, is a startling window into Florida's pre-Columbian history in the heart of a bustling metropolis, archaeologists say.

A cache of artifacts -- including shells, beads and pottery shards -- has persuaded some experts that the circle is likely the foundation of a Tequesta Indian building at the site of one of Miami's first trading posts founded by northern settlers.

But another, more intriguing theory also has been advanced: That the circle is a celestial calendar, perhaps made by a breakaway band of Mayas, the sophisticated Central American Indians who lived in the Yucatan, Belize and northern Guatemala.

"It looks like Stonehenge in negative. Instead of stones -- holes," said T. L. Riggs, a surveyor who has studied Mayan culture.

Whatever the relic turns out to be -- the site was uncovered in August and researchers are in the initial stages of identifying and dating the artifacts -- it is a vision of Florida past in the bedrock of a city built on glitter.

"It has generated more questions than answers," said Bob Carr, an archaeologist and director of Miami-Dade County's Historic Preservation Division, which is heading the archaeological dig at the site.

Historians expected to find Indian artifacts when bulldozers moved in to demolish the old Brickell Apartments and prepare the site for a new luxury tower. The patch of land at the mouth of the Miami River was known to have been a homestead and trading post for the Brickell family, early Miami settlers, in the 1870s.

The site lies in the shadow of a Sheraton hotel and is a stone's throw across the narrow river from a Hyatt hotel erected on the site of a Tequesta village. The native Indians inhabited the region when Ponce de Leon, the Spanish explorer, landed in Florida in 1513 seeking the Fountain of Youth. The Tequesta all but vanished because of war and disease following the arrival of the Europeans.

This summer, when the diggers scraped bedrock through a thick layer of landfill and midden -- the black earth formed from the refuse of previous occupants -- they uncovered a series of man-made holes in the form of an arc.

Riggs extrapolated the arc, etching a circle on the ground where he expected the rest of it might lie under the dirt. A backhoe dug along the outline and more holes emerged in the form of a perfect circle 38 feet in diameter.

The mysterious circle survived the construction of the Brickell Apartments unmarred. Work crews buried a septic tank in the middle of the circle without touching the holes. A sewer pipe sits beside the southern point.

"Nothing like this has ever been found in south Florida," said John Ricisak, a Miami-Dade historic preservation specialist who has worked at the site for months. "To my knowledge, if it is the foundation of a Tequesta structure of some sort, it would be the first hard evidence of one that's ever been documented archaeologically."

Although Ricisak and Carr believe the site is likely Tequesta, Ricisak said the celestial calendar theory would not be "as far out as it might seem."

"It would not be unprecedented," he said. "In the Old World, for example, there was Stonehenge."

Riggs, who spent years living in Central America and studying the Maya, theorizes that a group of Maya may have made their way to the U.S. mainland through the Florida Keys hundreds of years ago. Some of the holes in the circle were meticulously cut into the shapes of marine creatures, such as the manatee, turtle and dolphin, he said.

"This is unique in the world. I don't think anyone has ever discovered where glyphs have been carved into the ground," he said. "There will be a lot of doubters. This would be the first evidence of the Maya in Florida."

But Michael Coe, a Yale University professor and leading expert on Mayan culture, downplayed the likelihood that the circle is Mayan.

"I think the chances against it are tremendous. There has never been any Mayan artifact found in Florida," Coe said. "The Maya really stayed put. They never got up into the United States. There is no hard evidence that they went to the [Caribbean] islands."

Researchers have a number of puzzles to solve, Ricisak said. Stones appear to have been carefully placed in the holes at the eastern, western and southern points of the circle.

Large quantities of flint and two ax heads fashioned from basalt were found at the site. Neither occurs naturally in south Florida, and the two closest sources of basalt, a volcanic rock, are the Appalachian Mountains of eastern North America and the highlands of Guatemala, a site of Mayan settlements.

But Coe said the Maya did not use basalt. "They had much better stuff than that," he said.

Carr suspects that the circle may have been the foundation for posts that formed the structure of an "upper-level, elitist-type [Tequesta] house, a chief perhaps."

"We know that they could create structures," he said. "I find it difficult to believe that it's actually a calendar. But I don't have a hard time believing some knowledge of astronomy figured into the construction."

Carr points to the backbone of a shark perfectly preserved within the circle. "The shark has its head to the west and tail to the east, very much the way the Indians would put a human in the ground," he said.

Developers plan to build a 600-unit luxury tower on the property. The company did not return calls for comment. Despite the site's potential historic significance, Florida law would allow officials to halt construction only if the site turned out to be an ancient burial ground.