Silicon Valley's obsession with finding new ways to take to the sky is growing.

Google co-founder Sergey Brin has been building a giant airship inside a hangar in Mountain View, according to Bloomberg. The secret work is taking place at the NASA Ames Research Center, down the street from Google's headquarters.

Engineers have already built the airship's metal frame, which takes up most of the Ames Center's massive Hangar 2. While Google took over those NASA hangars in 2015, the secret project reportedly isn't a Google or Alphabet venture.

Bloomberg's anonymous sources say the project is being led by Alan Weston, the former director of programs at NASA Ames. Weston was previously a member of the Air Force and has in the past worked on a lunar spaceship.

During a 2013 interview, Weston spoke about a hypothetical airship that could haul 500 tons of cargo. It would be helium-based and would essentially "breathe," pulling in air from the outside as pressure within the craft shifted. The ship, he said, could land at places other than airports, and would be more fuel-efficient than a truck.

It's not clear whether that's the ship Brin is building. It's also not apparent whether he plans to monetize the project, or if it's simply something he's doing for fun. He wouldn't confirm the project to Bloomberg, saying only that he doesn't "have anything to say about this topic right now."

Brin is just the latest tech tycoon to get involved with building a flying machine, whether publicly or in secret. Fellow Google co-founder Larry Page personally has been funding two flying car startups: Zee.Aero, which launched in 2010 and has mostly remained out of the public eye, and Kitty Hawk, which also has been secretive, but released a video of its vehicle flying over water earlier this week.

Travis Kalanick evidently is eyeing the sky as well: Earlier this week, Uber announced it has partnered with NASA and the FAA as it tries to build a network of flying taxis. The company says it would like to have vehicles in the air by 2020. Eventually, it wants those vehicles to fly autonomously and to cost as little as a regular Uber ride.

A3, the Silicon Valley innovation lab for airplane manufacturer Airbus, is developing its own flying cars. The company claimed last year that it had chosen a design for the vehicles and that the first test flights were planned for 2017.

While flying vehicles have been a sci-fi fantasy for decades, big strides have been made in recent years. Still, some huge obstacles exist before they can become a reality. The vehicles wouldn't stand much of a chance of clearing regulatory hurdles without automated systems for avoiding deadly collisions, which some have cited as the most challenging part of developing flying vehicles.

Meanwhile, no country in the world currently allows automated drones to fly over urban areas. And developing in the U.S. might prove even harder than other countries: The FAA's strict rules have led some companies, like Amazon, to test their drone technology abroad. Getting any kind of flying vehicle out to the masses would likely require working closely with that organization--and the process could take a long, long time.

Still, the fact that some of the brightest minds in the world are looking upward is promising for the industry--even if some of those projects are destined to exist only to amuse their creators.