''It's a challenge,'' Dr. William C. Harris, the new president of Biosphere 2, said of the transformation as he sat in his office, with the Santa Catalina Mountains visible out his window. ''This facility was not designed for these kind of experiments.''

Most ecologic research is done outdoors, he noted, and no one has ever before tried to bring it inside on so vast a scale, replete with experimental controls and all the rigor that modern science can muster.

''You can think about doing experiments like you might do in chemistry or physics, where you change things, where you stress the system and see how it responds,'' said Dr. Harris, a physical chemist by training and most recently an official of the National Science Foundation, the Government's main agency for financing basic research.

''The potential is so significant that -- if we can turn this into an experimental tool -- it will resolve questions that cannot be answered otherwise. It will influence how we think about the rest of the world.''

Trucks and construction crews now swarm over the desert site as Columbia nears a milestone in its takeover. On Nov. 25, the Biosphere will open to the public for the first time, at least part of it.

The old habitation area of the Biospherians, as the eight men and women called themselves, has been sealed off from the rest of the domes and transformed into a visitor center full of exhibits on climatic change. It is the first of the subdivisions.

Inside, visitors can learn not only about how the planet is warming but can tour old Biospherian residences and learn something of the glass ark's woes, of how the crew lost weight, got sick and began to grow paranoid about food theft.