This idea of deep tunnels dug with bombs has intrigued me and I think it must have come from this cold war project. This article sums it all up quite well. tldr, we tried digging wide holes, we tried digging deep holes, we sent radioactivity all over the world, it's not a good idea (but if we could only direct a little tiny beam of antimatter we could dig a better hole...)

https://www.smithsonianmag.com/smart-news/the-us-once-wanted-to-use-nuclear-bombs-as-a-construction-tool-1957380/

The U.S. Once Wanted To Use Nuclear Bombs as a Construction Tool From digging a harbor to expanding the Panama Canal, how couldn’t nuclear bombs be used?

By Colin Schultz SMITHSONIANMAG.COM JANUARY 16, 2013 In 1962, the Atomic Energy Commission wanted to see how big of a hole they could make with a nuclear bomb. Enter, Project Sedan. The year was 1957. The Cold War was in full swing, and Sputnik was in the air. The U.S. was seemingly lagging behind in the technological arms race and needed to make a show, a display of power and prowess. Formed five months earlier by the U.S.’s Atomic Energy Commission, Project Plowshare, says Motherboard, was a project in which the nation’s scientists were supposed to find something useful to do with all the nuclear expertise they had acquired throughout World War II and its aftermath.

In what stands as the preeminent example of the high-stakes one-upmanship that fueled the Cold War, Sputnik’s launch put a ton of pressure on U.S. researchers to come up with a similar marquee scientific achievement. As historian Norman Chance explains, scientists at the Lawrence Radiation Laboratory suggested that using nuclear bombs as huge shovels would offer the “highest probability of early beneficial success” in the early stages of Plowshare. From 1961 through 1973, Project Plowshare saw 27 nuclear detonations. Many of these were at a test site in Nevada, says Motherboard, but some were a bit more experimental. In 1973, Project Rio Blanco, an operation under the banner of Project Plowshare,

“was an attempt to release 300 trillion cubic feet of natural gas under the Rocky Mountains by blasting apart caverns more than a mile deep with a trio of 33-kiloton bombs. It was the final of three attempts by Plowshare researchers to create what basically amounted to nuclear fracking.”

The project team wanted to blow a path for a railway line through California’s Bristol Mountains; they wanted to use nukes to expand the Panama Canal; and they wanted to use underwater explosions to carve out a harbor in Alaska. Plowshare’s first act was Project Gnome. In 1961, “the U.S. went ahead with Gnome, burying a 3.1 kiloton device over 1,100 feet deep in a massive salt deposit below New Mexico.” The goal was to see if underground nuclear explosions could be used to generated electricity. The result was to vent radioactive material to the atmosphere.

And, in the least productive but most destructive test, the scientists wanted “to see how big of a hole a nuclear bomb could make.” Motherboard: “It proved to be a really big hole.”

That test, Project Sedan, spewed radioactive fallout across four states, contaminating “more Americans than any other nuclear test.” Such a foolhardy project continued for so long, says Motherboard, in part because the public’s knowledge of the dangers of nuclear testing wasn’t quite keeping pace with the visions scientists had for the bombs’ uses. Eventually, though, the public caught on. In 1977, decades after it started and “amidst public uproar,” Project Plowshare was shut down.

About Colin SchultzColin Schultz Colin Schultz is a freelance science writer and editor based in Toronto, Canada. He blogs for Smart News and contributes to the American Geophysical Union. He has a B.Sc. in physical science and philosophy, and a M.A. in journalism.