On the afternoon of October 28, the water in Boothbay Harbor seemed to disappear, withdrawing toward the ocean, then all at once rushing back toward shore in a muddy swirl of currents and 4-to12-foot waves. Then it happened again, and again.

The ocean’s strange behavior was noted along the coast from Bristol to Cundy’s Harbor. Eyewitnesses described rapids going out to sea like a rushing river; torrents of water; beautiful, scary whirlpools.

Emergency management officials notified the National Weather Service, who checked with the West Coast and Alaska Tsunami Warning Center to find out if an earthquake had generated a tsunami. Bill Knight, a scientist on duty at the center at the time, confirmed that their global network of 350 seismometers did not pick up any sign of an earthquake.

The phenomenon was not a storm surge or tidal surge as initially reported. Amid much speculation, several solid theories remain and are being investigated by scientists around the country. Probable causes include a storm-driven wave event (a seiche or “meteoro-tsunami”) or an underwater landslide.

Dr. Joseph Kelley, a marine geologist at the University of Maine, thinks an above-ground landslide is unlikely. “If it was a landslide, you’d have to have a huge amount of material falling into the sea, and there is no bluff big enough to cause the waves like that.”

A bluff eroding into the sea, or an underwater sediment slump, could have caused the kind of waves seen in Boothbay Harbor: a drop in water level followed by surge, with a 15-minute period between waves. These events are known to occur in places like Alaska and Spain.

After reviewing tide and wave data, Dr. Juan Horillo, a tsunami expert at Texas A & M, discarded a local underwater landslide, since a nearshore landslide usually generates an initial large wave front with smaller subsequent waves. “Now, if this tsunami had been triggered by a relatively distant submarine landslide, it will be hard to tell,” he said.

Horrillo’s colleague at Texas A & M, Dr. Vijay Panchang, has developed a wave forecast model for the Gulf of Maine. “The kinds of waves I deal with are wind-generated, and this doesn’t look like that’s what happened,” he said. “These are not regular storm waves.” Panchang is chair of the Maritime Systems Engineering Department.

“A number of people are thinking it was meteorological in origin,” said John Jensenius, warning coordination meteorologist at the National Weather Service, who issued a statement about the event the following day.

During a sudden storm, a quick drop in air pressure (called a “bomb” by air traffic controllers) can cause an area of ocean to bounce or bulge upwards. These atmospheric disturbances generate barotropic or gravity waves in the open ocean. While these waves may have occurred in area too deep and open to be detected by the closest Gulf of Maine Ocean Observing System buoy, as a wave approached the wedge-shaped Boothbay Harbor, it would have grown in height with narrower and shallow conditions.

“There was a storm over southern New England, and we had some rather gusty winds at the same time,” said Jensenius. “Of course, we’ve had those things at other times without this kind of event. It’s kind of a puzzle, and you have to put the pieces together.” The pieces include the offshore storm, a dramatic wind shift, and the strong currents of a mid-ebb tide, all of which combined in a unique way in the vicinity of Boothbay Harbor, creating a seiche-like oscillation.

“The bathymetry of the ocean floor reflects and refracts the wave energy and can cause significant variation in tide levels along the coast, and rapid changes in water levels at a particular location, Jensensius reported in his statement. “These surges are quite unlike the much slower surges normally associated with coastal storms.” Coastal tide gauges showed unusual activity within a 15-minute period on the afternoon of October 28.

John Sowles, of the Maine Department of Marine Resources in Boothbay, saw from his office window what he described as an upwelling of water and strange currents with whitecaps stretching for hundreds of feet. Wind data he obtained from the Brunswick Naval Air Station record a rapid shift of 100 degrees.

Wind sculpts the sea surface into waves, but the current below the surface moves in the opposite direction. “In my mind, I picture the wind blowing in a southwesterly direction, with the undercurrent flowing northeast to replace the water being blown offshore. Then the wind shifts, forcing the water to switch directions,” Sowles described. The whole event took about 15 minutes, after which floats, docks, and a lobster boat had been ripped loose, causing an estimated $10,000-20,000 in damage. Had this all occurred at high tide, the damage could have been much worse.

Meanwhile, oceanographers and meteorologists continue to investigate the event, but as Jensensius said, “We may never know what really happened.”

Catherine Schmitt is the communications coordinator for Maine Sea Grant Program at the University of Maine.