What is this secret Google Building?

What is this thing? Google is building something secret and huge construction on a floating barge, in the San Francisco Bay.

A water-based data center? the structure stands more than four stories tall and is floating on a large barge, off shore from Treasure Island.

CNET speculates that inside the building a prototype floating data center is constructed. The center will use sea water to cool data centers and this could save the company a lot of money.

There is an other speculation according to gizmag that the building is meant to become a floating marketplace to promote Google Glass.