Researchers in Egypt have an opportunity to recover the most ancient air ever sampled, in a dig adjacent to the great pyramid of Cheops at Giza.

Not until six or seven years ago did scientists realize it was worthwhile to try to sniff out old air. Now a number of projects are under way to sample air from 20 years to thousands of years old.

The main point is to compare the Earth's present and past atmospheres in order to verify the theory that carbon dioxide is increasing, leading to a projected global-warming disaster in the next century.

Researchers in Switzerland have taken ice from Greenland and other spots, found air bubbles in the ice, then analyzed the air released from the bubbles. They believe they have air about two centuries old in their best bubbles.

Allen Ogard and colleagues at the Los Alamos National Laboratory have begun a search for air-trapping objects to do the same thing. The Los Alamos group believes some of the best objects involve sealed metal. Metals such as brass are unlikely to react with the air inside, Ogard said.

Currently, the group is popping open two-piece military buttons dating to the early 1800s. Other possibilities are old optical instruments with lenses sealing the ends.

The Egyptian search, if that country can get the funding to carry it out, will open a sealed limestone chamber adjacent to the largest and most famous of the Egyptian pyramids. The chamber is one of two known since 1952. The other proved to contain one of Cheops' cedar boats intended to carry the Pharoah to the afterlife.

The age of the boat -- and the air that workers said escaped in a rush of cedar aroma when the chamber was opened -- is about 4,600 years.