Billionaire entrepreneur Elon Musk seems like the ideal person to get excited about flying cars. The Tesla CEO has made a fortune in developing future-facing vehicles, but he’s rather downbeat about the idea that the skies could hold the solution to congestion woes. Instead, during an interview about his new tunnel-digging venture, he said the future of transport lies underground, naturally.

“Obviously, I like flying things, but it’s difficult to imagine the flying car becoming a scalable solution,” he told Bloomberg Businessweek. “If somebody doesn’t maintain their flying car, it could drop a hubcap and guillotine you.”

“Your anxiety level will not decrease as a result of things that weigh a lot buzzing around your head.”

Musk has talked before about how he sees tunnel digging as the way forward, and even though he’s explicitly tweeted that he’s “actually going to do this,” many were still unconvinced it wasn’t all an elaborate joke. But Musk is adamant: his new venture is real, it’s called the Boring Company, and it’s currently working on its first project to build a pedestrian tunnel in the SpaceX parking lot.

Bloomberg Businessweek's Musk-mole-themed cover. Bloomberg Businessweek

Traffic congestion is an undeniable issue in a lot of cities, but a lot of people in Silicon Valley see flying cars as the solution. Uber has published a white paper proposing a vertical takeoff and landing vehicle, ideal for confined cityscapes, that would work in conjunction with hired car rides at either end. The Silicon Valley branch of Airbus has developed a “Vahana” flying taxi prototype that would be ideal for Uber’s plans:

Airbus' Vahana. Airbus

Outside of Silicon Valley, Dubai has decided to get into the flying taxi business. At the World Government Summit on Monday, the city’s transportation authority revealed plans to introduce self-driving drones that carry passengers as soon as July. The initial launch will use the EHang 184 drone that can fly for 25 minutes carrying up to 220 pounds of weight, cruising at a speed of 37 mph around the city.

It’s not just ride-sharing where people are exploring the idea of a flying car. Google co-founder Larry Page has founded two startups committed to making the flying car a commercially available product.

The problem, as Musk sees it, is that a flying car would involve a lot of force to maintain its position in the sky. The people on the ground would bear the brunt of the produced wind, noise and debris. It’s also a far more technically complex solution to city congestion than tunnels. Cities already have tunnels. They work with existing vehicles.

With Musk’s vision, cities could have up to 30 layers of tunnels, ferrying various types of vehicle around. By digging down, cities would add greater infrastructure capacity without using precious horizontal space.

The first step is improving existing tunnel technology: Musk wants to bring the price of digging down by making more powerful boring machines that are capable of digging even while tunnel walls are being built. Right now, that’s not possible, but at least the Boring Company won’t need to worry about bigger problems like how to stop cars falling out of the sky.