Armchair scientists can venture 2 kilometers underground Tuesday morning to tour the newly completed Solar Neutrino Observatory (SNO). A live webcast  will whisk virtual visitors down a mine shaft near Sudbury, Ontario, through airlocks and decontamination chambers, to get a rare glimpse of the observatory itself: a 12-meter-tall, ultraclean vat soon to be filled with heavy water for detecting neutrinos streaming from the sun.
The $75 million project is designed to answer a vexing question: Why does half the sun's neutrinos seem to be missing? Solar models predict that the rate of one type of neutrino (the electron neutrino) should be twice as high as observed in current experiments. The leading explanation for the deficit is that the electron neutrino eludes capture by transforming into one of the other two kinds of neutrinos. By watching for flashes of light (and other signals) sparked when a neutrino plows into a heavy water molecule, SNO will be the first observatory to track all three neutrino types at once and pin down any subatomic chameleons.
Neutrino signals are notoriously faint, so the researchers have gone to great lengths to keep dirt from getting into the underground lair. (A smattering of radiation from naturally occurring elements can "blind" the sensitive detector.) But for Web surfers on the christening tour, SNO officials are waiving the showers and hair nets usually required for entry. Ditto for the hundred people expected to participate on the other end. "I'm keeping my fingers crossed we don't get any contamination," says SNO associate director Davis Earle.