Google Co-Founder is secretly working with the Silicon Valley startup, Zee.Aero which Larry Page has personally funded since it has set up shop next to Google’s HQ in Moutain View, California. The firm has also filed a patent for small electric vehicles that can land and take off vertically.

Page has invested more than US$100 million in Zee.Aero, according to the Bloomberg story. The company has nearly 150 employees and has expanded its operations to include an airport hangar in Hollister, California, where prototypes now are being tested. It also has a manufacturing facility at NASA’s Ames Research Center in Mountain View. Page’s involvement in flying-car projects has been kept intentionally secretive.

When Zee.Aero launched, Bloomberg reports, employees were restricted to the first floor of the company’s two-floor building. On the second floor was Page’s multi-billionaire man-cave, consisting of, among other things, a climbing wall, paintings, and a SpaceX rocket engine, given to him by friend Elon Musk. Zee.Aero employees didn’t know who their funder was, but suspected he must be very wealthy. He was referred to as GUS, the guy upstairs. Soon enough, Zee.Aero ran out of room on the first floor, and Page’s man-cave was dismantled to make room. Zee.Aero has two prototype aircraft in a hangar not far from its headquarters. The electric vehicles take “regular test flights,” according to Bloomberg. In a patent filed four years ago, the company says Zee.Aero’s aircraft has vertical takeoff and landing capability.

Page also personally backed up the startup Kitty Hawk last year which began it’s operations just down the street from Zee.Aero.

At Kitty Hawk’s helm is Sebastian Thrun, head of Google’s self-driving car program and founder of Google X, the research division of Google, according to 2015 business filings.