THE SPHINX,that familiar symbol of undeciphered mystery, is at it again. A Boston University geologist has taken seismic soundings at the base of the brooding monument of Giza and has discovered, he says, that the statue may be 6,000 or 10,000 years old rather than the 4,600 years most Egyptologists assume. The assertion is something of a shock to the field, partly because most of the people lavishing attention on the Sphinx in recent years have been preoccupied with the opposite question of how quickly it is falling apart. Several leading Egyptologists dismissed the notion of an older Sphinx out of hand, citing long-established studies. But geologist Robert Schoch says the locals reacted to his idea with a good deal less surprise. After all, the legend that the Sphinx is older than the Pyramids -- which is to say, older than just about anything visible thereabouts -- has been around a lot longer than modern archaeological studies.

Egyptologists mostly take it as settled fact that the Sphinx was carved about the same time as the Pyramids with which it shares the Giza Plateau and that its gentle, enigmatic face (minus a nose, a beard and other bits that have fallen or been knocked off over the centuries) is actually the likeness of a Pharaoh of the same period, Chephre. They point to corresponding materials and to indications that the Sphinx is connected to temples that honor Chephre. Prof. Schoch sees no contradiction here: It's well known that the upper parts of the human-headed, lion-bodied statue have been renovated and recarved many times. His findings concern the Sphinx's base, which, he says, show signs of "deep weathering" inconsistent with that of the supposedly contemporaneous pyramids nearby. The weathering, he suggests, harks back to a wetter, rainier climate at Giza and therefore an earlier age.

If his findings prove out, Prof. Schoch will join a long tradition of non-archaeologist outsiders who shook up large or small orthodoxies of archaeology -- the most famous being Heinrich Schliemann, the amateur who dug up Troy. More likely, he'll encounter the field's main occupational hazard: the truly interesting questions aren't resolvable. But for the Sphinx, at least, that uncertainty seems appropriate. The Sphinx may have lost some of its air of mystery as the Cairo suburbs crept up around its flanks and as preservation-minded worshipers covered it with funny-looking equipment. But the Egyptians don't refer to it as the "Father of Terror" for nothing. Those who worry that modern technology drains mystery out of the world should take heart that, in this case, it may be doing the opposite.