The sanctuary of Delphi, in southern Greece, was one of the most important religious sites of the ancient world. It consisted of a stone temple, dedicated to the god Apollo, set in a dramatic natural amphitheater. Inside the temple sat the Delphic Oracle, a specially trained woman who responded to questions with prophecy�wisdom directly inspired by the god himself. The oracles spoke there for a thousand years, a center of sacred power and political influence, until the center was finally closed in the year 392.

How the oracle worked was not a secret. She sat in a small underground room, breathing vapors from the ground and drinking water or inhaling mist from the warm spring beneath the temple. She entered an exalted state of mind and gave advice as if in a trance. Sometimes she didn't make much sense; sometimes her answers went over her questioner's head. On at least one occasion, she went into seizures and died.

Plutarch, who ran the temple for many years, wrote that the gases of the oracular chamber smelled sweet, like flowers. He also studied the history of the temple and made some shrewd observations. By his time, late in the first century CE, the oracle was not considered as powerful as in old times and the gases were not often present in the chamber. He surmised that the vapors came from a source deep in the Earth that was slowly losing its potency, and he thought that the great earthquake of 373 BCE had closed off some of the passages leading up to the surface.

Modern scientists have tended to doubt Plutarch's explanation, but it looks like their reasoning was wrong. The first serious excavations at Delphi, in the early 1900s, found no fissures and detected no gases, but that proved nothing. Fissures can be very small, and they can be sealed over the centuries by mineral deposits. And it was already known that the gases had disappeared. The mistake was in thinking about where the gases had came from�until recently, only hot vapors from volcanic sources were considered.

Also, the Delphi region is underlain by a large body of limestone. Limestone tends to become highly porous as groundwater dissolves its minerals�this is why caves are found almost exclusively in that rock type. And this is not just any limestone, but one containing bitumen, organic remains that give rise to underground oil and natural gas.

The research team, led by Jelle Z. de Boer of Wesleyan University, examined pieces of travertine�a limestone stalactite deposited by an ancient spring�to see what gases they might have trapped. They also looked for hydrocarbon gases in the modern spring water. The light gases methane and ethane were detected in measurable amounts.

But the telltale gas de Boer's team found was ethylene. They also found ethylene at other Greek "gas springs" and natural tar pits. Ethylene is an important industrial gas today, so we know a lot about it. First, it's an effective anesthetic that produces euphoria and excites the nervous system. Second, it has a sweet smell.

As a result, we have a persuasive explanation for how the Delphic Oracle worked physically. De Boer concludes, "Our research has confirmed the validity of the ancient sources in virtually every detail, suggesting their testimony on geology is of more value than has recently been held to be the case."

I think, too, that having a scientific explanation doesn't lessen my wonder at what the Delphic Oracle meant to the ancients. It puts me in awe of how strong the institution was and how well it worked. And I marvel too at the power science has to bring those long-dead people, whose culture roots our Western civilization, less distant from us.