The welcome mat is rustic: prairie grass flattened by a pickup truck.
To a visitor walking up a slight rise to the tracks' end at a pile of dirt and two makeshift worktables, it appears that no one is home. Suddenly, a hand hefting a bucket sprouts from the ground, vanishes, and is replaced by a human.
The human is Jim Mertz, a volunteer helping archaeologists who are trying to answer fundamental questions about the ancient inhabitants of this site, known as Cahokia.
Archaeologists estimate that at a time when the Normans were consolidating their hold on a newly conquered England, central Cahokia covered five square miles, held more than 100 earthen mounds ranging from the mundane to the monumental, and was home to as many as 20,000 people.
Greater Cahokia appears to have embrace communities with dozens of additional mounds in what are now St. Louis and East St. Louis, forming what many archaeologists say was the heart of the Mississippian culture.
Yet like the ancient Anasazi, whose cultural center in New Mexico's Chaco Canyon peaked between AD 850 and 1250, the Mississippians at Cahokia melted away, perhaps to merge with what became more modern groups. By 1400, only the mounds remained as silent testimony to a once-thriving cultural center.
Researchers are trying to pin down the factors that led to the rise and fall of Cahokia, as well as details about how the society evolved over time. Recent discoveries of Mississippian communities on the Illinois plateau, which rises above the Mississippi River plain some five miles east of Cahokia, are prompting some researchers to overhaul their views of how the society was organized and how it maintained its cohesion.
Today, notes University of Illinois archaeologist Timothy Pauketat, Cahokia mounds represent "a linchpin in the archaeology of eastern North America."
Mr. Mertz is scraping his way into one of them, known as Mound 34.
He clambers up a stepladder and out of a rectangular trench with corners as sharply defined as the crease on a sailor's dress whites. He empties his bucket onto a makeshift sieve, then sweeps his gloved hand back and forth, breaking up clods and sifting through the dirt in search of archaeologists' "black gold" - charcoal.
"If we can get enough charcoal, we can get a good date" on the layer of soil that contained the specimens, says Mertz, referring to radiocarbon dating techniques.
Getting a "good date" on the mound's contents is critical to answering several questions, notes John Kelly, a research associate at Washington University in St. Louis and the lead scientist on the Mound 34 dig. With less that 1 percent of the historic site excavated, researchers are still trying to pin down the order in which the mounds were built and rebuilt. The sequence is expected to help define the changes that took place in the society's structure over time.
More intriguing is the potential role Cahokia played in developing cultural and religious symbols and ideas that it exported to more distant groups. Dr. Kelly notes that Mound 34 appears to have been a ceremonial site centering on war and hunting. Unique shell artifacts found in 1956, but less rigorously dated, also have cropped up in mounds at Spiro, Okla., he says.
This has led to a collaboration between Kelly and Northwestern University's James Brown to gain a better handle on what Dr. Brown calls "a burst of craft specialization" that led to art forms now called the Southeastern Ceremonial Context. Cahokia could have been the hub for that art form, researchers say.
While Kelly and colleagues are digging at Cahokia, Dr. Pauketat has been exploring sites on the Illinois plateau. This past summer, his team finished work on a site that he called "mind-blowing."
According to Pauketat, central Cahokia's population underwent what he's termed a "big bang" during a 50-year period that began about 1050. It skyrocketed from an estimated 1,000 to 2,000 residents to about 16,000. Old-style homes, built with a single central post, were replaced with houses that used upright log walls set into trenches.
The major issue now, he says of Cahokia's evolution during this period, is to "figure out what it is that's being created and how long it lasts."
Five to 10 years ago, Pauketat continues, a debate over how Cahokia was organized appeared to end with the notion of Cahokia as a state or major city yielding to the idea that it was merely a large chiefdom, similar to those found throughout the Southeast. New evidence during the past few years, however, may rekindle the discussion.
"Look at the central mound complex," he says, referring to a set of mounds surrounding a 40-acre grand plaza, and marked by the granddaddy of them all, Monks Mound. The tiered mound rises 100 feet above the landscape, covers 14 acres, and contains more than 22 million cubic feet of earth. Excavations at its summit revealed that it once hosted structures, perhaps housing the leader's family. This central complex alone "is 10 times bigger than any chiefdom in the Southeast," Pauketat observes.
Discoveries on the nearby Illinois plateau appear to bolster that view, he continues. Four years ago, some researchers held that the plateau had nothing to offer in terms of sites, reinforcing the notion that the Mississippians lived in settlements on the river plain in a manner loosely linked to central Cahokia's chiefdom.
Yet Pauketat and graduate student Susan Alt are finding a variety of villages from the period on the plateau, many with specialized roles, such as agriculture, fiber production, or beadmaking.
"We are finding big villages out there, and they are producing Cahokian stuff in ways that show they are linked pretty closely" economically to central Cahokia, Pauketat says.
Work on the latest village, some 15 miles southeast of St. Louis at Shiloh, Ill., ended this summer. Based on its finds, the team suggests that the original village boasted features such as a large central residence and a temple-like building not often found in other regional villages.
Pauketat and Ms. Alt speculate that it would have been a farming village feeding Cahokia as well as an administrative center. Colleagues who have worked on Mayan sites argue that it may be time to think of Cahokia "more like a founding city," he says, whose influence spread up and down the Mississippi.