When it surfaced two months ago, the notion of Teslas whizzing through underground tunnels between San Antonio International Airport and downtown seemed fanciful.

Now, there’s a sign the idea may have gained some traction.

The Boring Co., a tunneling firm backed by billionaire Elon Musk, has been talking to local leaders about building an underground transportation loop in San Antonio. Musk is the CEO of electric-vehicle maker Tesla, as well as the founder and CEO of SpaceX.