This underground art is relatively unknown beyond a circle of World War I scholars and enthusiasts, as well as village mayors and landowners, many of whom Gusky has spent years getting to know.

For 25 years, Art on the Underground has been filling the Underground with art. It’s what they do, and they do it well. They’ve put Mark Wallinger, Zadie Xa, David Hockney, Laure Prouvost and ...