Google co-founder Sergey Brin is said to be secretly building an airship inside Hangar 2 at the Nasa Ames Research Center.

People familiar with the project told Bloomberg that Brin has been fascinated by airships. He decided to build one about three years ago after he saw photos of USS Macon, an airship built by the US navy.

The metal skeleton of the craft has apparently been constructed but it is not clear if the project is just a hobby or if Brin wants to turn it into a business.

In an emailed statement, Brin told Bloomberg, "Sorry, I don't have anything to say about this topic right now."

The airship project is reportedly led by Alan Weston, the former director of programmes at Nasa Ames. He has worked in the US Air Force where he did engineering work for the Star Wars missile defense system. In 1989, he oversaw one of the first tests of Star Wars, which was aimed at destroying Russian missiles midair. He then joined Nasa and worked on several projects.

In a 2013 radio interview, Weston talked about his plans to make airships more fuel efficient than planes.

"New airship technologies have the promise to reduce the cost of moving things per ton-mile by up to an order of magnitude. It depends on the size of the airship. A larger airship can reduce costs a lot more than a smaller ship, but there's design of a class of vehicles that can lift up to 500 tons that could be actually more fuel-efficient than even a truck," Weston explained in the interview.

"And so the way that works is that the helium in the main envelope is taken and stored in bags inside the airship at a slightly higher pressure. As you do that, air is taken in from the outside into essentially like lungs that are attached in the side of the vehicle. So the analogy of breathing is a good one. And the overall lift of the vehicle is equal to the weight of the air that is being displaced by the helium. And as you change that, you can control the amount of buoyancy that the vehicle has," said Weston.

With the technique, the airship could carry 500 tons without requiring for a ballast.