PHOENIX -- Archaeologist Gina Gage pulls a fragment of red pottery decorated with black stripes from the ground and begins to explain how it might fit into the history of one of America's most advanced and mysterious ancient cultures.

Then her voice is drowned out and the shard trembles as a 747 takes off a few yards away and another of the seven jets lined up on the taxiway revs up.

Gage and her team have been excavating a roughly 500-year-old Hohokam farm site before it gets paved over in an expansion of Phoenix's Sky Harbor International Airport.

Despite its reputation as a city born after the discovery of air conditioning, modern Phoenix sits atop relics from the past 2,000 years. What's being done to preserve that heritage from urban growth makes the city a leader nationwide in conservation.

Phoenix is one of a handful of U.S. cities -- including Boston, New York, Alexandria, Va., and St. Augustine, Fla. -- to have a full-time archaeologist on staff. The position in Phoenix was established in 1929 and is thought to be one of the first.

"To have a city archaeologist, that's an excellent example of how the state is working," said James Walker of the Archaeological Conservancy, a nonprofit group dedicated to preserving archaeological sites nationwide. "Arizona is leading the country in these areas."

Constant activity

As the Phoenix archaeologist, it is Todd Bostwick's job to oversee nearly 700 sites, including Gage's dig, in a sprawling metropolis covering 470 square miles.

"It's not like it's some quiet little space where you do what you do," Bostwick said. "You've constant activity all around. And we go from 10-block projects to five feet of sewer line."

The area's archaeological remains are seldom eye-catching, consisting mostly of adobe buildings and canals built by the Hohokams, who lived in the Salt River Valley from A.D. 1 to A.D. 1450.

But they constitute pieces in an unsolved puzzle: What caused the Hohokam and other Southwest people to suddenly disappear or devolve to very simple societies in the 15th century?

"Most of what's excavated by archaeologists is information, not the spectacular findings that people think of," said Linda Mayro, the cultural resources manager for Pima County.

The Hohokam turned the harsh desert into farmland by building about 1,000 miles of canals. Then, about A.D. 1450, they vanished.

Archaeologists still wonder why.

Zoning law

That makes it even more important to preserve any clues and in Phoenix, zoning law is designed to do it.

Regardless of whether the land to be developed is public or private, if the project includes rezoning the developer has to do an environmental impact review.

"As development has accelerated, more sites are being destroyed, which makes others all the more valuable," Mayro said. "Once they're gone, they're gone forever."

Gone, like the Hohokam, whose name is modern Tohono O'Odham for "those who have gone."