As the devout among the ancients knew well, nothing spices up a boring sermon like having your own sacrifice pit parked in front of your church. Throw in a secret tunnel to the death chamber, and you've got a churchgoing experience that no suburban mega-church, no matter how many good parking spots it offers, could ever match.

An ancient Temple of Apollo located amid the ruins of Hierapolis, the "sacred city," in Western Turkey suggests such attractions may have been something of a franchise among temples during the Roman era. Hierapolis was a Greek city famed for its hot springs that the Romans took over in 133 B.C. Apollo, the Sun god, was the chief deity of the city, and Italian researchers from the University of Lecce reveal some of the inner workings of the temple there in the current Journal of Archaeological Science.

The temple's ruins rest on a plateau running along the eastern side of the Menderes River, which itself runs along a geological fault. The fault produced Hierapolis' hot springs, popular with the bath-loving Romans, and also poisonous gases. Those poisonous gases, in this case it seems suffocating quantities of carbon dioxide, appear to be one of the secrets of the Temple of Apollo.

The temple, dating to the 3rd Century A.D, sat atop a monumental staircase and "near it there is an underground cavity called the Plutonion," says the study. A hole nearly 30 feet wide, surrounded by a fence, the Plutonion was "covered by a thick mist, making it impossible to see inside," study co-author Giovanni Leucci said by email. "The air outside the fence is quite clear, and when no wind is blowing there is no danger in approaching it, but any living creature that enters the hole dies instantly."

A tough place to do research, in other words. But starting in 2001, a team led by Leucci and his colleague Sergio Negri, along with the late Ivo Richetti, undertook a series of ground-penetrating radar and electrical resistance studies of the Plutonion and the temple. "The Plutonion was used in the past to perform animal sacrifices and only the eunuchs of the Temple of Cybele were able to spend time within the cavern without being affected," Leucci says. (Cybele was an Earth Mother-type goddess associated with caverns whose most devoted followers castrated themselves and were regarded as belonging to a third gender by the ancients. Cybele's cult also revolved around a theme of death and rebirth, which may explain the attraction of going spelunking in a poisonous death trap.) The eunuchs likely covered their heads with four sacks of cloth, Leucci says, which held a pocket of air that allowed them to survive for few minutes inside the Plutonion.

The Plutonion, named after Pluto, the god of the underworld, was known to widen as it descended. The researchers hoped to learn whether it was connected to the temple itself.

It does. "The survey carried out of the Temple of Apollo clearly suggests the presence of a man-made structure," Leucci says, namely a tunnel about 8 to 14 feet under the temple. And a room about 13 feet across seems to lie at a similar depth beneath the temple. The find suggests that priests likely retrieved sacrifices from the pit, prepared them in an underground room, and displayed them in the temple above as part of a religious ritual that may have resembled an elaborate stage magician's trick.

The search also turned up that an unsuspected geologic fault runs under the temple grounds. Such faults may be a hallmark of Apollo's temples, as well as the famed Oracle of Delphi, whose visions some suspect came from underground fumes. Other temples of Apollo in Turkey were home to oracles and they were built over active springs, such as those at Didyma and Claros.

So the key to temple success way back when may have rested on a rather earthly concern, access to a geologic fault, something even harder to find than a good parking spot is today.