The Southern Environmental Law Center released the report Oct. 21. It was written by Chris Groves, a hydrogeologist at Western Kentucky University, who examined the proposed Music City Loop and Nashville’s karst geology: porous limestone shaped by water into underground caves and voids.

The company plans two tunnel projects. One would run 9.5 miles between downtown and Nashville International Airport, beneath 8th Avenue South and Murfreesboro Pike. A second, 2.9-mile route would run from West End down Broadway and beneath the Cumberland River to Nissan Stadium. Passengers would travel through the tunnels in vehicles driven by operators, in a rideshare-like setup.

Groves’ report says portions of the routes cross karst-prone rock near the Cumberland River, Mill Creek and other waterways, despite the company’s stated intention to avoid known karst formations. Such cavities can be difficult to locate before excavation, the report says, particularly when exploratory boreholes are spaced about 500 feet apart.

The report describes potential hazards including sudden flows of water or mud, damage to tunnel-boring machines, ground subsidence and sinkholes. Groves also identified large underground cavities near the airport route, including one about 13.5 feet across and a cave with passages measuring as much as 30 by 40 feet.

The Boring Co.’s mitigation plan describes using drilling, geophysical testing and advance probing to detect underground hazards. But Groves said the plan does not adequately explain the methods or how they reflect experience from other tunnels built in similar conditions. A separate assessment by a company contractor rated the risk of encountering karst as “low to moderate.”

The SELC report also identifies nine contaminated sites near the planned route, based on state environmental records. It highlights the former Aladdin Industries plant on Murfreesboro Pike, where soil, groundwater and other materials have been contaminated with trichloroethylene, other solvents and metals. Trichloroethylene is a carcinogen.

Trey Bussey, an attorney with the environmental law center, said the company has not provided enough detail about how it would manage contaminated water if construction encountered it, or who would pay for damage caused by a tunnel-related incident. He raised questions about a boring machine becoming stuck beneath Mill Creek or a sinkhole disrupting downtown traffic and threatening nearby buildings.

In an environmental assessment issued in 2025, The Boring Co. concluded the project would not significantly affect Nashville’s geology or other key resources. The assessment said the company would “attempt to avoid tunnel boring in areas where karst development is identified.” The company acknowledged Nashville’s geological challenges in 2025 and developed a mitigation plan, according to records obtained through a public-records request. CEO Steve Davis called the geology an engineering problem that was relatively straightforward to solve.

The Boring Co. did not respond to a request for comment on the new report. The findings raise questions for Nashville residents and public officials about how the company would protect groundwater and nearby infrastructure, and how it would respond if tunneling uncovers hazards that could not be avoided.