AI-assisted summary The Boring Company is attempting to build a tunnel system in Nashville, despite the city's challenging geology which has hindered past tunneling projects. Nashville's terrain, composed of hard rock and erosion-prone limestone, presents difficulties for traditional tunneling methods. The Boring Company CEO acknowledges the geological challenges but expresses confidence in their technology's ability to overcome them. Previous attempts to build a subway system in Nashville have been deemed cost-prohibitive due to the region's hard bedrock. The potential for a subway system in Nashville has historically felt out of reach to city leaders. Tennessee’s tough terrain — marked by variations of erosion-prone limestone and other hard bedrock — stymied engineers' prospects of digging infrastructure beneath the surface for decades. Now, The Boring Co. is testing whether its cutting-edge tunneling technology can succeed where traditional methods have failed. It remains to be seen whether Elon Musk's tunneling venture will succeed, but if it does, it could open the door to high-speed tunnel systems throughout the southeast. What makes tunneling in Nashville so difficult, geology experts explain The path from downtown to BNA runs through the Outer Nashville Basin, an area known for its diverse bedrock. Here, the city sits on a mix of soft limestone and rock that The U.S. Geological Survey describes as "cherty Mississippian-age formations," meaning very hard and dense sedimentary rocks that resist erosion. "Limestone is generally a stable sedimentary bedrock material with strength parameters that are favorable for tunneling," said Jakob Walter, founder and principal engineer at Haushepherd, a California-based civil engineering firm. "Limestone is however fairly soluble when compared to other rock materials, and can dissolve over long period of time when exposed to water." That prolonged exposure can cause the rock to fracture, forming caverns and caves — a common feature of Middle Tennessee's geology, Walter said. "Unexpected encounters with these features while tunneling can result in significant construction delays and potential instability of the excavation," he said. If left unaddressed, voids can trigger sinkholes as the ground collapses into the underlying cavities. In prevention, engineers must fill gaps with grout or sand during the excavation process. Subsequently, nearby buildings require close monitoring. "In urban locations, structures at the ground surface should also be constantly monitored with robotics total stations or similar surveying equipment to identify any early signs of movement or distress," Walter said. Boring Co. CEO calls city's rock an 'engineering problem' The Boring Co. itself acknowledged the geological challenges it faces in Nashville during its official announcement of the Music City Loop tunnel from downtown to the Nashville International Airport on July 28. "It's a tough place to tunnel, Nashville. If we were optimizing for the easiest places to tunnel, it would not be here. You have extremely hard rock, like way harder than it should be," President and CEO Steve Davis said. "It's an engineering problem that's fairly straightforward to solve." Davis said The Boring Co. sees two challenges in Nashville terrain: the durability of the rock and the inconsistent depth to bedrock. Davis didn't elaborate on exactly how his engineers plan to solve it apart from his simple belief in the company's technology — more specifically the Prufrock, Boring Co.'s version of a tunnel boring machine. Civil engineer Sri Kumar said private investment into solving infrastructure problems — like the challenge of safely tunneling through limestone — has become critical. “We always need more innovation, and frankly, more money in infrastructure,” Kumar said. “There are so many infrastructure projects to be done in the nation that both public funding and labor resources simply cannot keep up with them.” Kumar is president and CEO of Mount Juliet-based engineering consulting firm Connico, which has contracted on numerous Nashville airport projects over the last 30 years. Into the archives: Why past Nashville tunneling projects failed Since the 1970s, Nashville has sporadically considered moving Nashville transit underground, but each time, city officials ultimately dismissed the idea. A group of Metro leaders traveled to Oakland, Calif., in 1972 to study its Bay Area Rapid Transit, which combines subways and an above-ground rail system. The Tennessean reported then that “Metro’s limestone shelf is considered to preclude any subway system.” Jo Federspiel, the head of the Nashville Metropolitan Transit Authority in 1988, also ruled out a Nashville subway system as the MTA evaluated options for mass transit improvements. Federspiel's reasoning was simple: “Just because of the bed of limestone we sit on.” In 2017, the city reconsidered, but its NashvilleNext report once again found that underground transit was not a feasible option. "The geology and geography of the Middle Tennessee region prohibits the construction of subway-type mass-transit systems,” the NashvilleNext report stated. “The hard bedrock and preserved, forested hills of the region make underground transit cost-prohibitive.” Gov. Bill Lee said this deal is different from past propositions because it puts the entire burden of cost onto The Boring Company. After more than 50 years of consideration, transit may finally head beneath the city as early as 2026. Hadley Hitson covers business news for The Tennessean. She can be reached at hhitson@gannett.com. To support her work, subscribe to The Tennessean. Stuart Dyos covers the automotive industry in Middle Tennessee and beyond. He can be reached at sdyos@gannett.com. To support his work, subscribe to The Tennessean. This story was updated to add a gallery.