Ryan Wheeler, Florida's state archaeologist, and other experts who have studied the Circle think the holes were dug by the Tequesta Indians to support wooden posts for a tribal center, or other important structure. But it's has been theorized to be everything from a celestial observatory to a landing pad for aliens.
Whatever it was, this much is certain: There's nothing like it on the continent. Authenticated as prehistoric, it is on the National Register of Historic Places for the clues it could yield about the complex society developed by the Tequestas, a small tribe who were foraging in the Everglades and Biscayne Bay before the building of the Parthenon in Athens.
Yet visitors to the park, which won't open for at least a year, will see only … an 8-foot replica.
The Circle has a great web site devoted to it. It's got a virtual tour with close-up photographs and a description of what each photo is showing. It also shows many of the artifacts plus summary reports for the 1999 excavations by week. Excellent resource and shows what can and should be done with these sorts of sites. It's easy to navigate and informative.