Hunting down the magnetic monopole.

The search for fundamental particles that have only one magnetic pole is now focused on a huge monopole detector nearing completion in Italy's Gran Sasso National Laboratory, located about 60 miles east of Rome, beneath the Apennine mountain range. This rectangular, concrete-and-iron structure, called the Monopole, Astrophysics and Cosmic Ray Observatory (MACRO), stretches nearly the length of a football field and already serves as the largest underground detector of cosmic rays. The bottom level of the two-layer detector of nearly finished, with several sections already collecting data. The top level should begin operating next year.

The project's main goal is to find a magnetic monopole. So-called grand unified theories, which mathematically tie together the electromagnetic, weak and strong forces, predict that magnetic monopoles -- if they exist -- would have a large mass and move much more slowly than other subatomic particles. These theories also suggest that the Big Bang created numerous monopoles, but only a small number would have survived from that time. "There's not any evidence whatsoever that there are lots of monopoles out there, and lots of evidence to the contrary," says Henry J. Frisch of the University of Chicago, who has led monopole searches in the past but is not associated with MACRO.

"We know they have to be scare if they exist," says Richard M. Heinz of Indiana University in Bloomington, a physicist on the MACRO team. "But at least with a detector the size of MACRO, we have a shot at finding one."

Just in case there is no monopole out there waiting to be found, MACRO is versatile enough to serve several other needs. "It turns out that while you're looking for magnetic monopoles, you can do a lot of other science," Heinz says. Although designed for seeking monopoles, the detector can also pick up signals from neutrinos as well as cosmic rays.