Seems like an answer to a question that nobody asked, and likely another dead-end program. To be tactically useful, it would seem like these tunnels would have to be constructed very quickly:

"The goal of the Underminer program is to demonstrate the feasibility of rapidly constructing tactical tunnel networks that enable secure, responsive resupply. A tactical tunnel network could provide secure logistics support infrastructure, such as pre-positioning supplies in advance of an operation or providing ongoing resupply as troops move through an area. The ability to rapidly bore tactical tunnels could be helpful in contingency operations such as rapid ammunition resupply or rescue missions."

https://www.darpa.mil/program/underminer