How badly does Google want to keep under wraps a mysterious project taking shape on a barge in San Francisco Bay? Badly enough to require U.S. government officials to sign confidentiality agreements.

At least one U.S. Coast Guard employee has had to sign a non-disclosure agreement with the internet giant, said Barry Bena, a coast guard spokesman who was later instructed not to talk to the media about the project because of "commercial confidentiality."

Another person who would only identify himself as an inspector for a California government agency had to do the same.

USA Today and the San Jose Mercury News reported that the coast guard confirmed that they had visited the barge on Wednesday but could not say why other than to specify that it was not for a fire or medical emergency.

Several coast guard representatives told the papers they had been instructed by supervisors not to discuss the project.

"Once the project is completed, we will be releasing information," Lt. Joshua Dykman told the San Jose Mercury News.

Four barges registered

Moored in the shadow of the Bay Bridge off Treasure Island, a former military base, the nondescript barge is stacked several storeys high with white shipping containers, and sprouts what appear to be antennas on top. The hulking structure, half shrouded in scaffolding, has stirred intense speculation in the Bay Area since reports of its existence surfaced late last week.

The floating structure docked at Portland Harbour in Portland, Maine looks to be composed of about 63 shipping containers welded together, a local newspaper reported. (Shawn Patrick Ouellette/Portland Press Herald/Associated Press)

Technology website CNET theorized that the vessel might be a floating data centre that will house banks of computers. Google was granted a patent in 2009 for just such a floating data centre, the site reported.

Local TV station KPIX said the barge is intended to serve as a floating retail store for Google's Glass wearable computer device — although its external appearance, at least thus far, doesn't suggest such a purpose.

Adding to the mystery, a second similar barge was spotted in Portland, Maine. A total of four barges have been registered to a Delaware corporation called By and Large LLC, an apparent nod to the fictitious company Buy and Large from the hit animation film WALL-E.

The barges are registered under the names BAL0001, BAL0010, BAL0011 and BAL0100, the numbered portions of which, observers speculate, likely refer to the binary code equivalents of "one," "two," "three" and "four." Two are in the San Francisco area — one of which houses the shipping-container structure — one is in Portland and the fourth is so far unaccounted for.

Google itself is keeping mum, refusing even to acknowledge its affiliation with the vessels.

Coast guard is 'fully aware' of what's on barge

On the east coast, a spokesman for the local coast guard told the Portland Press Herald newspaper that work on the Maine barge will begin after its San Francisco counterpart is complete. Ensign Connan Ingham​ told the paper that coast guard officials had been asked by the barge owner not to talk about it but that "they are fully aware of what's on that barge."

The paper dispatched reporters to the Portland Harbour to get a closer look at the barge docked there and reported that the structure on the barge appears to be composed of 63 metal shipping containers welded together.

"The structure is four containers high, four containers wide and four containers long," the paper reported. "One container is slanted at a 45-degree angle to create a ramp​.

"There are doors on each of the upper three floors on both ends of the structure. There are also narrow windows. It's possible to look through some of the windows and see windows on the other side, indicating that the sides of some of the containers have been removed to create open floor space."

The structure was assembled in New London, Conn., according to the paper, and its journey to the harbour was documented in a photo posted on Facebook by the company whose tugboat towed the barge into the harbour.

Spent year preparing project

Secrecy is a standard business practice in Silicon Valley, where technology companies such as Apple go to great lengths to keep their latest gadgets under wraps and a constellation of blogs compete to reveal highly prized details.

It was a phenomenal production. None of them would tell us anything. - Bob Jessup, construction worker

But the concealment effort surrounding the barge is in another league. Chain-link fences and security guards encircle a pier and a couple of nearby buildings on the island, which sits between San Francisco and Oakland.

A California state inspector, who said he had business in the hangar-like Building 3 where some of the early construction took place, told Reuters he had to surrender his mobile phone and sign a confidentiality agreement in order to enter.

A close-up of the structure docked at Treasure Island, where the pier is encircled by chain-link fences and security guards. (Jeff Chiu/Associated Press)

Bob Jessup, a construction company superintendent who works in a building across the street, said Google spent the past year working on the project. He said the company fenced off a wide area and brought in at least 40 welders a day, who worked around the clock and refused to say a word.

"They wouldn't give up any of the information," Jessup said. "It was a phenomenal production. None of them would tell us anything."

He said they worked on the inside and the outside of the shipping containers, outfitting them with electronics — "very hush hush" — and then loaded them onto the barge with a crane. They put sides on the containers, with glass windows in some of them. They had to weld them very precisely so they could stack, Jessup recounted.

Jessup said he could not imagine that Google would try to use the floating vessel as a retail outlet. "Who's going to want to climb up in there?" he asked. "It's really ugly."

Barge will need permit if it stays

Some nearby property on Treasure Island has been subleased to the same firm to which the two barges are registered. Representatives of By and Large could not be reached for comment.

Larry Goldzband, the executive director of the San Francisco Bay Conservation and Development Commission, told Reuters his agency has had several meetings with Google officials about the barge in recent months. Yet, the company provided little information other than telling him that the vessel will be used for "general technology purposes," he said.

Google "could not give us a specific plan of any kind," not even whether they intended the barge to move or stay in one place, Goldzband said. If the barge remains in place for an extended period of time after its construction is completed, it will require a permit from the BCDC, he said.

"We've asked counsel to get us as much information as soon as they can, so that we can continue the discussion," Goldzband said, referring to Google's law firm.