During a Reddit AMA on Tuesday, engineers from Hyperloop One explained why they prefer to build tunnels above the ground, unlike Elon Musk’s The Boring Company, which plans to dig below-ground hyperloop tunnels in the coming years.

“Will all hyperloops be above ground? Or will you explore below ground options? Thinking it would be easier to go under cities then through?” asked one user during the AMA.

“It is preferable to stay above ground, except when necessary in very dense urban areas,” replied civil engineer Ismaeel Babur. “Below ground options are definitely possible; however, above ground systems are generally cheaper, faster to build, and easier to control construction, etc.”

Babur’s response seems to indicate that none of the three systems Hyperloop One hopes to have in service by 2021 will be built in a dense urban area. That’s not too surprising, since Hyperloop One has been conducting all of its testing above the ground in a remote desert north of Las Vegas, on a development track it calls “DevLoop.”

boring company elon musk above-ground below-ground
A 500 meter tube installation built by Hyperloop One at DevLoop.

However, not everyone agrees that above-ground tunnels are the way to go, including the guy who invented the concept for hyperloop in the first place, Elon Musk.

Musk founded The Boring Company — which is now a Hyperloop One competitor — as tunnel-digging enterprise. He’s made it clear that he likes below-ground tunnels because they are “immune to weather” and “quiet.” The Boring Company website also argues that such tunnels improve city congestion, are “out of sight,” and provide good protection during earthquakes because “the tunnel moves uniformly with the ground, in contrast to surface structures. Additionally, a large amount of earthquake damage occurs from falling debris.”

Of course, tunneling into the ground is slow work, and even Musk admits that the Boring Company needs to find a way to improve its digging speed. But for all of its talk about above-ground tunnels being “faster to build,” Hyperloop One isn’t progressing so quickly, either. In fact, a Boring Company spokesperson told Inverse on Monday that Musk decided to build his own hyperloop because he doesn’t think that other companies are moving fast enough.

When the initial projects of both Hyperloop One and The Boring Company have been realized — which will likely be in just a few years, if Hyperloop One manages its 2021 goal and the latter catches up —we’ll be able to judge for ourselves whether hyperloop tunnels are best above or below the ground.

Photos via Hyperloop One