Is Nessie merely a bad case of the shakes?

Sightings of the Loch Ness Monster have variously been attributed to ancient marine reptiles that somehow survived extinction, uncommonly large sturgeon, and too much Scotch whiskey. Now, an Italian scientist contends that the original source of the monster's legend, as well as the basis for many of the modern encounters with the supposed beast, may be seismic activity.

At about 25 miles in length, up to 1 mile in width, and about 800 feet in depth, Loch Ness is one of the largest and deepest lakes in Scotland. It lies atop the Great Glen fault, which generates three or four moderate earthquakes each century, says Luigi Piccardi, a structural geologist at the Center for the Study of the Geology of the Apennines in Florence.

Most Nessie sightings have occurred in the northernmost portion of the loch. It's probably no accident that this region also harbors the most active portion of the fault, Piccardi said last week in Edinburgh at a joint meeting of the Geological Societies of America and London.

Seismic activity has produced waves on the loch before. A major earthquake near Lisbon, Portugal, on Nov. 1, 1755 was felt from northern Africa to Finland. It caused a giant sloshing wave, called a seiche, on Loch Ness.

The sighting that spawned the legend of Nessie occurred in the 6th century when St. Columba, an Irish monk who converted much of Scotland to Christianity, supposedly prevented a great beast from devouring a man in the lake's water. Perhaps tellingly, Piccardi notes, this encounter happened little more than a mile from the spot where a major quake would strike in 1901.

To date, more than 3,000 sightings of the Loch Ness Monster have been reported. Many are hoaxes. Most of the other descriptions don't include details of the presumed monster itself, Piccardi notes. Instead, they typically chronicle a large commotion in the water or odd waves. Piccardi says that many of these can probably be explained by bubbles of gas released by the fault underlying the loch. Also, large gas pockets generated by rotting vegetation trapped in the lake's sediments could be shaken loose by seismic activity too small for people to feel.

Some of the more vivid sightings include observations of low humps breaking the surface. Piccardi says that these could be waves resulting from a focusing of seismic energy rumbling beneath and through the loch's waters.

Many Nessie sightings probably describe real phenomena that simply have been misinterpreted, says Stephen J. Cribb, a geologist with Carraig Associates in Inverness, the city near the northern end of Loch Ness. "People have been suckered by natural phenomena for ages," he notes.

Although seismic activity could explain many Nessie encounters, Cribb says no scientific explanation is likely to quash either a belief in the monster or the tourism industry that tales of Nessie have stimulated. "For true believers, you only need one unexplained sighting to preserve the mystery," he says.