SOUTH PORTLAND — Robin Drummond was making a cup of Earl Grey tea Thursday when he saw something unusual out his kitchen window. He grabbed his binoculars and ran out to the terrace.

Sure enough, off Portland Head Light was the barge carrying the four-story building he had seen in the news. He got his mother, who got her iPhone, and they sped to Spring Point Ledge Lighthouse for a better look.

“Everything that comes in and out of the channel, we are on the case,” said Grace Drummond, standing next to the lighthouse with her 20-year-old son.

The “mystery” building was headed for Cianbro Corp.’s Rickers Wharf Marine Facility off Commercial Street in Portland, where it is scheduled for a significant amount of interior work, including the installation of technological equipment, before it is delivered to its owner.

Peter Vigue, chairman and CEO for The Cianbro Cos., said he is not at liberty to identify his client or divulge how the structure will be used.

The structure was assembled on a barge in New London, Conn. Pulled by a Portland tugboat, the Rowan W. McAllister, it left New London about noon Wednesday and arrived off Fort Williams Park in Cape Elizabeth just before 6 p.m. Thursday, when it was met by a second tugboat.

The Drummonds have seen a lot from their home on Danforth Cove: two plane crashes, a man overboard and, just this summer, a pleasure boat that caught fire right off their beach.

Had there not been a shroud of mystery surrounding the building, they might not have raced down to the breakwater Thursday to get as close as possible to the barge.

“The reason this is so interesting is because it’s so secret,” said Grace Drummond.

She thinks it has something to do with the government.

The Day, the daily newspaper in New London, tried to find out what was being built on the barge but learned nothing from the construction company. It then filed a Freedom of Information Act request with the Coast Guard. Still nothing.

In Portland, waterfront business owners and marine officials said they don’t know what the structure’s owner plans to do with it.

Readers have made some wild guesses on the Portland Press Herald’s website.

One reader guessed that it’s a “zombie apocalypse shelter,” and another said it must be Section 8 housing. Another said the structure is a tar sands oil processing plant, and one person theorized that it could be a floating prison, similar to one in New York City.

“It’s a new restaurant, just what Portland needs,” said another Press Herald reader.

Tourists on the breakwater at Spring Point around sunset Thursday watched the odd-looking vessel as it made its way there from Portland Head Light, where many of them had just been.

“I didn’t see it a half hour ago,” said Jaap de Haas, a vacationer from Holland who guessed that it’s an oil platform or container ship.

“Maybe it’s a new dormitory for your pharmacy school,” joked Larry Hitchcock, who was visiting his daughter, a student at the University of New England.

Sisters Michelle and Kathie Hodson, who took a day trip from Boston to check out Maine’s lighthouses, said it looked like a giant mobile home. “Like a double-wide, but longer,” said Kathie Hodson.

They took out their cameras.

“I’m going to take a picture to say we saw a building on a boat,” Michelle Hodson said.

Around 6:30 p.m., the barge stopped. The tugboats switched places. The sky was dark when the barge got going again 15 minutes later, and the Drummonds were the only people left.

Once the boats passed the lighthouse, the mother and son walked back down the breakwater. By the time they got to the end, the vessels’ lights were barely visible. The Drummonds said they planned to go check it out at Rickers Wharf at some point.

“We’ll see what we can see,” Robin Drummond said.

They were probably still driving home when, at 7 p.m. sharp, the traffic light on the Casco Bay Bridge turned red and bells began to ring.

First in line at the light was a South Portland city bus. The driver and a couple of passengers got out while the drawbridge was up. They didn’t notice the building, which was barely visible from the bridge in the dark.

Chris Mucigi, however, was curious. He got out of the bus, which he was taking from class at Southern Maine Community College to his home in Portland.

“What is it?” he said, leaning over the fence to look as it passed under the bridge. “It has windows and everything.”

Ten minutes later, traffic started moving again. Drivers crossed the bridge with no idea what had just passed – even if they had seen it.

Leslie Bridgers can be contacted at 791-6364 or at:

lbridgers@pressherald.com

Dennis Hoey can be contacted at 791-6365 or at:

dhoey@pressherald.com