The sun was tracking its low arc across the sky, illuminating the stone structures embedded in the earth. At the top of the rise Mr. Hill and I ducked our heads to enter the L-shaped Oracle Chamber. Dark and damp inside, the carefully constructed warren is half buried underground, and includes the sacrificial table, a four-and-a-half ton grooved slab of granite thought to be 4,000 years old. What ancient rituals were performed here, and by whom, remains a subject of debate.

“I’ve never visited a site  anywhere  that combines standing stones with stone chambers,” Mr. Hill said. “That’s a certainly a large part of my fascination.”

Two days later I returned to America’s Stonehenge for a consultation with David Brody, a local lawyer and mystery novelist who shares Mr. Hill’s belief that European visitors built this place. Mr. Brody pointed out that the complex of cairns, walls, chambers and huts was encircled, at a distance of approximately 100 yards, by notched “sighting stones” that lined up with the sunrise and sunset on important dates like the summer and winter solstices.

“There were no calendars or almanacs” 4,000 years ago, said Mr. Brody, 47, a hearty, goateed man in hiking boots and a flannel shirt. By marking the rising and setting sun on certain days, the people who built the structures “knew when to plant and harvest crops, when to launch their ships and when to pray,” he said.

Ground mists are fuming up from the lowlands as we ascend the hill. “The colonial argument never made sense to me,” Mr. Brody said, squatting beside one of the chambers to indicate how the rising sun would strike a certain stone on the inner wall. Noting that colonial settlers would not have been dependent on astronomical events to track the days of the year, he said: “There’s too many of these to write off as coincidence. Sure, there’s evidence that the Native Americans used them as sweat lodges. But when you have elaborate stone structures like these, there’s a lot of reuse when a new civilization comes along.”

Departing the raw New Hampshire woods, I encountered a honeymooning couple, Mike and Georgia Sasso, both 23 and history lovers from West Point, Miss. “It makes you feel what it must’ve been like, living way back then,” Mr. Sasso said.

With a giggle, Ms. Sasso said: “It was a little scary. What I saw was  it felt ancient.”

Driving home in the rain, I recalled something that Mr. Hill told me. “So many things in the world today, we have to figure out exactly what it is. Here’s something that’s three, four thousand years old, and we don’t know who constructed it and how they used it. More than that we’re never going to know.”

IF YOU GO

America’s Stonehenge (105 Haverhill Road, Salem, N.H.; 603-893-8300, stonehengeusa.com) is open year round. Admission prices are $9.50 for adults, $8.50 for 65 and older, $6.50 for children ages 6 to 12 and free under 5.