Four storeys tall, 250ft long and 72ft wide, there’s little doubt that Google’s latest idea is a big one.

The only problem: nobody knows what it is yet.

The mysterious structure, swaddled in scaffolding and floating on a vast barge in San Francisco Bay, is composed of shipping containers welded together, painted white with several narrow slits for windows and topped with 12 spires that may be antennae.

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The waterborne monolith has been linked to Google, yet the search giant has refused to confirm it owns the structure or reveal its purpose.

The barge, which is generating feverish speculation in San Francisco and Silicon Valley, is registered to a Delaware firm, By and Large LLC.

The mystery deepened this week with the discovery of another, near-identical barge 2,700 miles away on the east coast of the US, in Portland, Maine. This is said to have appeared in Portland’s harbour on 11 October and is also registered to By and Large.

Local media reports suggest a Maine-based company, Cianbro, has been making secret interior and electronics modifications to the structure. Cianbro would not provide any details of its client or the work, but revealed the barge is destined to leave Portland once it is completed. A comprehensive investigation of the San Francisco barge by the Cnet website determined its most likely purpose was as a massive, sea-going computer server. In 2009, Google was granted a patent for a “water-based data centre”.

Bob Jessup, a construction professional whose workplace is adjacent to the facility on Treasure Island in San Francisco Bay, where the structure was assembled, told Reuters the project had been underway for a year.

The area was fenced off, he said, while the shipping containers were outfitted with electronics equipment and then loaded on to the barge with a crane. He estimated that about 40 welders worked on the structure, but would not reveal any details.

“It was a phenomenal production,” Mr Jessup said. “None of them would tell us anything.”

Over the weekend, the Bay Area news station Kpix reported that the barge might be a mobile store for a new line of high-tech eyewear, Google Glass, designed to travel from city to city via the coast and rivers.

The search giant is sufficiently concerned about keeping its latest venture a secret, however, so that even US government officials have been legally prevented from disclosing any details about the barges and their contents. At least one coast guard employee and an inspector for a California state government agency have signed non-disclosure agreements with Google.