A few weeks ago, the sun set on the bottom of the world, not to rise again for six months. Temperatures at the pole are now around minus 70 degrees Fahrenheit. By the depths of winter in July, they will have crept toward 90 below, and even colder. The already super-thin, ultra-dry air will have shed a few more wisps of water vapor.

In short, conditions are becoming ideal--at least for the increasing number of physicists and astronomers who go to the end of the Earth for their research.

The National Science Foundation's polar outpost has become an astrophysical hot spot chiefly because it is the one place on the planet where certain kinds of radiation can be observed through the atmosphere. Many intriguing celestial objects can be seen only in non-visible light. Such light--infrared, millimeter and microwave, whose wavelengths are 100 to 10,000 times longer than the wavelengths your eyes see--are effectively filtered out by warmer or denser air.

Sometimes an object's visible light gets swallowed en route to Earth by intervening cosmic dust; only the longest wavelengths penetrate the haze. (For exactly the same reason, sunlight is red at dawn and sunset, when the rays have to travel farthest through the atmosphere; only the longest orange and red wavelengths remain unscattered.) That is the case for newborn stars in their gas-clotted "stellar nurseries," or for things near the center of our galaxy.

Unfortunately, those long wavelengths--including telltale radiation from clouds of atomic carbon, which is essential to understanding the physics of star formation--are easily absorbed or scrambled nearly everywhere in the Earth's comparatively warm and restive atmosphere, whose incessant stirring makes stars appear to twinkle. Ditto for the cosmic microwave background signals left over from the earliest years after the Big Bang. Scientists observe that radiation to study the shape and distribution of matter in the primordial universe.

At the pole, however--with an elevation of 9,300 feet, an air density about two-thirds of Washington's, and the coldest sky in the world--atmospheric and heat interference is minimal.

For measuring long-wavelength radiation, "the worst day here is better than the best day at Mauna Kea," Hawaii, home of the big Keck telescope, said Robert Lowenstein of the University of Chicago, who heads the Advanced Telescopes Project at the pole station. "A small telescope here can do things a telescope five times larger would do in temperate latitudes."

So the NSF's collaborative Center for Astrophysical Research in Antarctica (CARA) supports three long-wave telescopes in the "dark sector" away from the half-buried, 160-foot-wide geodesic dome of the main pole station.

For example, with even a 24-inch-diameter telescope such as SPIREX (South Pole Infrared Explorer), "you can see into our galactic center," said astronomer Al Fowler, and study the structure of other galaxies as well. "Another important area is star formation. You have to be able to look into the cloud to see the protostars. Do they just form from dust or some kind of cataclysm?" ("In a sense, we are stellar pediatricians," said Roopesh Ojha of the Harvard-Smithsonian Center for Astrophysics, who works on the nearby AST/RO telescope.)

Ironically, as cold as the pole is, the telescope detectors have to be vastly colder--only a few degrees above absolute zero--to eliminate thermal "noise." Much of the equipment is highly automated, and data are sent back to the United States via satellite, but a few scientists, such as Ojha, spend the entire sunless winter here tending the gear.

The most exotic instrument at the pole, however, works day or night, summer or winter. It is an entirely new kind of telescope called AMANDA (for Antarctic Muon and Neutrino Detector Array). Scientists hope to use it to identify distant sources of ferociously high energy, such as "active" galactic centers or mysterious gamma-ray "bursters." Those entities also give off all sorts of radiation, as well as various charged particles. But the former gets absorbed in interstellar dust, and the latter can curve as they pass through various magnetic fields, making it hard to determine their point of origin.

So instead of gathering photons (the units of light or other electromagnetic radiation collected by conventional telescopes), AMANDA is designed to detect neutrinos. "It's the first telescope since Galileo that is really different," said physicist Steven W. Barwick of the University of California at Irvine.

Wraithlike, nearly massless neutrinos, produced by numerous high-energy processes such as nuclear fusion and streaming through everything in space, interact only very rarely with ordinary matter. Several million billion of them are shooting through your body every second, as little affected by their surroundings as "a bullet through a rainstorm," according to University of Wisconsin physicist and AMANDA collaborator Francis Halzen.

But every once in a while one of them bangs squarely into a particle in some atom's nucleus. This produces a fat cousin of the electron called a muon that flies off at high speed in the same direction as the original neutrino, and gives off bright blue light as it does. That light--Cerenkov radiation, the photon equivalent of a sonic boom--is a distinctive sign of a neutrino smashup.

AMANDA records them using about 400 downward-looking light detectors buried over a mile deep in the polar ice. At that depth, the pressure is so great that even tiny air bubbles (which could scatter the Cerenkov photons) have been squeezed out of the ice, and it is extremely clear. Nor are there any of the biological light sources that plagued earlier neutrino detectors using sea water.

Of course, there are a lot of neutrinos out there--about 100 million times as many as all the protons and neutrons in the universe--and plenty of them collide with atoms in the atmosphere, producing a constant drizzle of muons. So to weed out only the highest energy neutrinos that definitely come from distant cosmic sources, AMANDA needs an absolutely huge filter. Fortunately, one is conveniently available: the Earth itself. AMANDA looks straight down through the center of the planet. Virtually any muons coming from that direction must be the cosmic variety, which carry about a million times as much energy as the humdrum atmospheric neutrinos.

After drilling holes--some more than a mile deep--with jets of boiling water, the AMANDA team lowered 10 strings of light detectors into the ice on cables. After a day or so, they were frozen in place. By tracking the path of Cerenkov light through this array, AMANDA scientists reported earlier this year that they can determine the direction of an incoming neutrino within a couple of degrees. With this proof of principle in hand, they hope eventually to expand the detector to an "ice cube" grid 1 kilometer on a side to improve data volume and accuracy.

"Timing is everything," said Barwick. "We have to know everything to the nanosecond. That's one of the reasons that you can't do this experiment anywhere else, because the temperature elsewhere changes all the time. And so do the lengths of the cables," thanks to thermal expansion and contraction. "But here the ice is always the same: minus 50 Celsius."

CAPTION: LOOKING THROUGH THE EARTH AT THE SKY (This graphic was not available)

CAPTION: AMANDA researchers examine an optical module before lowering it into the transparent Anarctic ice.