An alien invasion? A secret installation? Or just Google…

From the fuss they have caused, you would think they had appeared overnight: vast, shining structures bobbing gently on the waters of San Francisco Bay and in the harbour at Portland, Maine.

Four storeys tall, 250 feet long and 72 feet wide, there’s no doubt the barges have made a splash (sorry). Even closer inspection doesn’t give much away – the structures are made of layers of anonymous shipping containers painted white, with small slits for windows, and what seem to be antennae on the top.

So, what are they? Alien ships? Futuristic floating prisons? Or some new sinister scheme cooked up by the NSA?

Sadly the truth is probably more prosaic than that, and – as with many of the kooky headlines which have come from the US in recent years (self-driving cars, trying to live forever, and glasses straight out of Minority Report) – it’s to do with Google.

The rumour is that the structures are floating data centres for the internet giant. Back in 2009, Google had a patent approved to build structures of this type, explaining that in the event of something like a natural disaster, back-ups may be needed for local computing infrastructure.

A key plus of launching the data centres on the waves is that sea water can be used cheaply and efficiently to cool the servers, and wave energy could even be used to power them.

It would also explain the secrecy, the chain-link fences surrounding the construction and the “hush hush” atmosphere from Google. The company itself is hardly known for being loose-lipped over new applications; and combine this with the fact that data centres are almost always kept extremely quiet because of the security risk if they were targeted, and the secret begins to make sense.

Is the game up for Google?

There are more clues too. The barges are registered as owned by a Delaware company called “By and Large LLC”, a possible reference to a company in the endearing robot film, Wall-E. And their names are in binary code – BAL0011 in Maine, BAL0010 in San Francisco.

But the game was really up for Google when a US coastguard spokesman, Barry Bena, admitted that at least one official had been forced to sign a non-disclosure agreement with Google on the issue. The company itself has declined to comment, even on whether it has anything at all to do with the vessels.

Read more on data, secrecy and the brave new world in the Data Baby project

A construction company superintendent in San Francisco spilled a bit more information, saying that Google had been working on the project for the past year with a team of at least 40 welders a day.

Bob Jessup, who works across the street, said: “They wouldn’t give up any of the information. It was a phenomenal production. None of them would tell us anything.”

Larry Goldzband, the executive director of the San Francisco Bay Conservation and Development Commission, said 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. There is some speculation the barges could also be used as storage or even as retail outlets for Google.

There’s an element of irony here as well, although Google probably won’t be amused – one of its secret operations exposed by a US government official on the day that the technology company is arguing for greater protection for secrets after described itself as “outraged” upon finding out it has been targeted by the NSA.

But – just in case it turns out to be secret prison after all – we’ll not say too much more about that, in case they Google it.

Watch below: a video of the Portland barge from the local paper, the Portland Press Herald.