For centuries, the Tequesta Indians were here, right where the Miami River flows into Biscayne Bay and the manatees and tugboats hurry by, long before the appearance of the big, bleached-white banks and hotels of downtown.

What they left behind, discovered only last year on a waterfront demolition site, was a circular formation of great mystery that seemed to fire emotions around the world. It was quickly dubbed the Miami Circle, and efforts to preserve it united a variety of groups in this often fractious city. To supporters, the discovery gave America a new archaeological cachet, and Miami something it had never entertained before: a vision of a distant past.

Now, the ongoing rescue of the Miami Circle has reached a critical point. Miami-Dade County Mayor Alex Penelas and others need to nail down the final $2 million of a $20 million payment due Tuesday; without it, the prime waterfront property could revert to a developer.

"I'm guardedly optimistic. I'm making a public appeal everywhere I go because this is so important," Penelas said this week.

Penelas may have averted a crisis--at least for the time being. In a memo to county commissioners in preparation for their meeting Monday, Penelas said the Trust for Public Land has offered to make a loan to complete the purchase, the Miami Herald reported yesterday. But the commissioners will have to approve the loan, and fund-raising efforts will have to continue.

An attorney for the owner, Brickell Point Ltd., said he is certain that Miami-Dade County will come through with the court-ordered payment. "There's nothing to say," Toby Brigham said Wednesday. "The court has entered an order, and I trust the parties will honor that order."

Otherwise, he said, the owner will be free to proceed with original plans for the property--the construction of twin residential towers more than 40 stories tall.

The Miami Circle, or at least the artifact-rich site on which it sits, could date back 2,000 years, archaeologists said. A perfect circle of low, carved limestone, about 38 feet in diameter, it was uncovered by county archaeologists after six apartment buildings on the site had been demolished--and just before developer Michael Baumann was to begin construction on the twin towers to replace them.

Immediately, some supporters imbued the circle, and its surrounding field, with a mystical significance, noting how some of the alignments squared with the sun. Others rather breathlessly proclaimed the discovery "America's Stonehenge." A stream of faxes, telephone calls and e-mails poured in from people around the world urging its preservation, and a motley collection of protesters gathered daily in a campaign to "Save the Circle."

The Inter-Tribal Council of the Five Civilized Tribes, which unites the Cherokee, Chickasaw, Choctaw, Creek and Seminole nations, joined the fight, passing a resolution of support for "our sacred religious sites" and condemning those who would allow the circle to be lost "for the sake of redundant development."

The Miami-Dade County Commission filed an eminent domain lawsuit in February that allowed it to take the property in exchange for payment of its fair-market value--$26.7 million. This was done after Baumann had suggested he would pay to have the formation moved to another spot, an offer that brought fresh howls of protest.

Supporters rejoiced that the circle appeared to be saved. But when it came to raising the $20 million down payment, contributions only trickled in. Most of the big money so far has come from state and county funds, although no taxpayer dollars are being used. The protesters also drifted away, leaving behind a chain-link entranceway draped in signs and strips of cloth and a makeshift altar.

Despite the last-minute trepidation, county officials and preservationists involved in the effort said the experience has been a good one--and a rare experiment for Miamians.

"While the case has the familiar ring of pitting preservationists against developers, what is unusual here is that that has never happened in Miami, a city only a century old that has had only a fleeting love affair with its history, and has rarely reached past the economic glitter and tinsel of the South Beach Art Deco District," said archaeologist Robert S. Carr, one of the discoverers of the circle and a consultant to Miami-Dade on the project. "The circle has become the anchor and resonance of Miami's lost heritage."

Carbon dating of artifacts found on the site suggests it is at least 2,000 years old. The items provide a clearer picture of life among the Tequestas.

According to Carr and others, the group existed as early as 500 B.C., but reached its heyday between 1200 A.D. and 1600 A.D. The Tequestas were comparable to Native Americans of the Pacific Northwest in their water-based lifestyle, busy with fishing and canoeing.

Explorer Ponce de Leon made note of the Tequestas in 1513, when they may have numbered 20,000, spread over an expanse of South Florida that stretched from Key West in the south to Palm Beach in the north to the Everglades in the west. But by the mid-1700s, an English writer who ventured here found only 300 Tequestas left, and most of them were eventually shipped off to Cuba.

"They were never conquered by the Spanish," Carr said. "What did conquer them was the germ--smallpox."

On a clear morning this week, as workers hurried into the surrounding banks and guests checked out of the neighboring hotels, county archaeologist John Ricisak and state archaeologist Ryan Wheeler sifted through the artifacts they have found so far. There was a dark-brown scrap of pottery; a bead made of a lead-like substance from the Great Lakes region, suggesting the Tequestas were involved in some long-distance trading; and an arrowhead, indicating perhaps that Tequestas, although not known as warriors, were prepared to defend themselves.

"There's definitely a lot of material here," said Wheeler, who is preparing a report for Gov. Jeb Bush (R) that says 70 percent of the 2.2-acre site could yield valuable artifacts.

Although plenty of animal bones have been found, he said, there have been no discoveries of human remains.

Both archaeologists believe the circle itself was the foundation of some kind of important Tequesta structure, perhaps a meeting house or temple. Holes carved into the limestone could have been used for supporting poles; there may have been a thatched roof. The capacity was about two dozen people.

If the $20 million payment goes through as planned, the site could eventually become a historical park, perhaps with a museum on its west end. The archaeologists would approve.

"It's a very, very unusual situation," said Ricisak, the county's field director for the project. "Archaeologists are used to working in obscurity, not with television helicopters overhead and people lined up along the gate talking to you with bullhorns.

"This, I think everybody recognizes, is a once-in-a-lifetime event," he said. "I don't know if the county is going to make a habit of stepping in like this, but I'm thankful for it. Because I don't think they made a mistake."